orphan/unborn: add to the glossary and use them consistently

To orphan is a verb that denotes the act of getting on an unborn
branch, and a few references to "orphan branch" in our documentation
are misuses of the word.  They caused end-user confusion, which was
made even worse because we did not have the term defined in the
glossary document.  Add entries for "unborn" branch and "orphan"
operation to the glossary, and adjust existing documentation
accordingly.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/advice.txt

@@ -140,6 +140,6 @@ advice.*::
 		Advice shown when a fast-forward is not possible.
 	worktreeAddOrphan::
 		Advice shown when a user tries to create a worktree from an
-		invalid reference, to instruct how to create a new orphan
+		invalid reference, to instruct how to create a new unborn
 		branch instead.
 --

Documentation/git-checkout.txt

@@ -215,7 +215,7 @@ variable.
 	below for details.
 
 --orphan <new-branch>::
-	Create a new 'orphan' branch, named `<new-branch>`, started from
+	Create a new unborn branch, named `<new-branch>`, started from
 	`<start-point>` and switch to it.  The first commit made on this
 	new branch will have no parents and it will be the root of a new
 	history totally disconnected from all the other branches and

Documentation/git-switch.txt

@@ -171,7 +171,7 @@ name, the guessing is aborted.  You can explicitly give a name with
 	`branch.autoSetupMerge` configuration variable is true.
 
 --orphan <new-branch>::
-	Create a new 'orphan' branch, named `<new-branch>`. All
+	Create a new unborn branch, named `<new-branch>`. All
 	tracked files are removed.
 
 --ignore-other-worktrees::

Documentation/git-worktree.txt

@@ -99,7 +99,7 @@ command will refuse to create the worktree (unless `--force` is used).
 If `<commit-ish>` is omitted, neither `--detach`, or `--orphan` is
 used, and there are no valid local branches (or remote branches if
 `--guess-remote` is specified) then, as a convenience, the new worktree is
-associated with a new orphan branch named `<branch>` (after
+associated with a new unborn branch named `<branch>` (after
 `$(basename <path>)` if neither `-b` or `-B` is used) as if `--orphan` was
 passed to the command. In the event the repository has a remote and
 `--guess-remote` is used, but no remote or local branches exist, then the
@@ -234,7 +234,7 @@ This can also be set up as the default behaviour by using the
 
 --orphan::
 	With `add`, make the new worktree and index empty, associating
-	the worktree with a new orphan/unborn branch named `<new-branch>`.
+	the worktree with a new unborn branch named `<new-branch>`.
 
 --porcelain::
 	With `list`, output in an easy-to-parse format for scripts.

Documentation/glossary-content.txt

@@ -312,6 +312,12 @@ This commit is referred to as a "merge commit", or sometimes just a
 [[def_octopus]]octopus::
 	To <<def_merge,merge>> more than two <<def_branch,branches>>.
 
+[[def_orphan]]orphan::
+	The act of getting on a <<def_branch,branch>> that does not
+	exist yet (i.e., an <<def_unborn,unborn>> branch).  After
+	such an operation, the commit first created becomes a commit
+	without a parent, starting a new history.
+
 [[def_origin]]origin::
 	The default upstream <<def_repository,repository>>. Most projects have
 	at least one upstream project which they track. By default
@@ -695,6 +701,18 @@ The most notable example is `HEAD`.
 	object,
 	etc.
 
+[[def_unborn]]unborn::
+	The <<def_HEAD,HEAD>> can point at a <<def_branch,branch>>
+	that does not yet exist and that does not have any commit on
+	it yet, and such a branch is called an unborn branch.  The
+	most typical way users encounter an unborn branch is by
+	creating a repository anew without cloning from elsewhere.
+	The HEAD would point at the 'main' (or 'master', depending
+	on your configuration) branch that is yet to be born.  Also
+	some operations can get you on an unborn branch with their
+	<<def_orphan,orphan>> option.
+
+
 [[def_unmerged_index]]unmerged index::
 	An <<def_index,index>> which contains unmerged
 	<<def_index_entry,index entries>>.