Documentation/git-config.txt: add SCOPES section

In a subsequent commit, we will introduce "protected configuration",
which is easiest to describe in terms of configuration scopes (i.e. it's
the union of the 'system', 'global', and 'command' scopes). This
description is fine for ML discussions, but it's inadequate for end
users because we don't provide a good description of "configuration
scopes" in the public docs.

145d59f482 (config: add '--show-scope' to print the scope of a config
value, 2020-02-10) introduced the word "scope" to our public docs, but
that only enumerates the scopes and assumes the user can figure out
what those values mean.

Add a SCOPES section to Documentation/git-config.txt that describes the
configuration scopes, their corresponding CLI options, and mentions that
some configuration options are only respected in certain scopes. Then,
use the word "scope" to simplify the FILES section and change some
confusing wording.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-config.txt

@@ -297,23 +297,20 @@ The default is to use a pager.
 FILES
 -----
 
-If not set explicitly with `--file`, there are four files where
-'git config' will search for configuration options:
+By default, 'git config' will read configuration options from multiple
+files:
 
 $(prefix)/etc/gitconfig::
 	System-wide configuration file.
 
 $XDG_CONFIG_HOME/git/config::
-	Second user-specific configuration file. If $XDG_CONFIG_HOME is not set
-	or empty, `$HOME/.config/git/config` will be used. Any single-valued
-	variable set in this file will be overwritten by whatever is in
-	`~/.gitconfig`.  It is a good idea not to create this file if
-	you sometimes use older versions of Git, as support for this
-	file was added fairly recently.
-
 ~/.gitconfig::
-	User-specific configuration file. Also called "global"
-	configuration file.
+	User-specific configuration files. When the XDG_CONFIG_HOME environment
+	variable is not set or empty, $HOME/.config/ is used as
+	$XDG_CONFIG_HOME.
++
+These are also called "global" configuration files. If both files exist, both
+files are read in the order given above.
 
 $GIT_DIR/config::
 	Repository specific configuration file.
@@ -322,28 +319,67 @@ $GIT_DIR/config.worktree::
 	This is optional and is only searched when
 	`extensions.worktreeConfig` is present in $GIT_DIR/config.
 
-If no further options are given, all reading options will read all of these
-files that are available. If the global or the system-wide configuration
-file are not available they will be ignored. If the repository configuration
-file is not available or readable, 'git config' will exit with a non-zero
-error code. However, in neither case will an error message be issued.
+You may also provide additional configuration parameters when running any
+git command by using the `-c` option. See linkgit:git[1] for details.
+
+Options will be read from all of these files that are available. If the
+global or the system-wide configuration files are missing or unreadable they
+will be ignored. If the repository configuration file is missing or unreadable,
+'git config' will exit with a non-zero error code. An error message is produced
+if the file is unreadable, but not if it is missing.
 
 The files are read in the order given above, with last value found taking
 precedence over values read earlier.  When multiple values are taken then all
 values of a key from all files will be used.
 
-You may override individual configuration parameters when running any git
-command by using the `-c` option. See linkgit:git[1] for details.
-
-All writing options will per default write to the repository specific
+By default, options are only written to the repository specific
 configuration file. Note that this also affects options like `--replace-all`
 and `--unset`. *'git config' will only ever change one file at a time*.
 
-You can override these rules using the `--global`, `--system`,
-`--local`, `--worktree`, and `--file` command-line options; see
-<<OPTIONS>> above.
+You can limit which configuration sources are read from or written to by
+specifying the path of a file with the `--file` option, or by specifying a
+configuration scope with `--system`, `--global`, `--local`, or `--worktree`.
+For more, see <<OPTIONS>> above.
 
+SCOPES
+------
 
+Each configuration source falls within a configuration scope. The scopes
+are:
+
+system::
+	$(prefix)/etc/gitconfig
+
+global::
+	$XDG_CONFIG_HOME/git/config
++
+~/.gitconfig
+
+local::
+	$GIT_DIR/config
+
+worktree::
+	$GIT_DIR/config.worktree
+
+command::
+	GIT_CONFIG_{COUNT,KEY,VALUE} environment variables (see <<ENVIRONMENT>>
+	below)
++
+the `-c` option
+
+With the exception of 'command', each scope corresponds to a command line
+option: `--system`, `--global`, `--local`, `--worktree`.
+
+When reading options, specifying a scope will only read options from the
+files within that scope. When writing options, specifying a scope will write
+to the files within that scope (instead of the repository specific
+configuration file). See <<OPTIONS>> above for a complete description.
+
+Most configuration options are respected regardless of the scope it is
+defined in, but some options are only respected in certain scopes. See the
+respective option's documentation for the full details.
+
+[[ENVIRONMENT]]
 ENVIRONMENT
 -----------