Some better parse-options documentation.

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

parse-options.h

@@ -22,6 +22,41 @@ enum parse_opt_option_flags {
 struct option;
 typedef int parse_opt_cb(const struct option *, const char *arg, int unset);
 
+/*
+ * `type`::
+ *   holds the type of the option, you must have an OPTION_END last in your
+ *   array.
+ *
+ * `short_name`::
+ *   the character to use as a short option name, '\0' if none.
+ *
+ * `long_name`::
+ *   the long option name, without the leading dashes, NULL if none.
+ *
+ * `value`::
+ *   stores pointers to the values to be filled.
+ *
+ * `argh`::
+ *   token to explain the kind of argument this option wants. Keep it
+ *   homogenous across the repository.
+ *
+ * `help`::
+ *   the short help associated to what the option does.
+ *   Must never be NULL (except for OPTION_END).
+ *   OPTION_GROUP uses this pointer to store the group header.
+ *
+ * `flags`::
+ *   mask of parse_opt_option_flags.
+ *   PARSE_OPT_OPTARG: says that the argument is optionnal (not for BOOLEANs)
+ *   PARSE_OPT_NOARG: says that this option takes no argument, for CALLBACKs
+ *
+ * `callback`::
+ *   pointer to the callback to use for OPTION_CALLBACK.
+ *
+ * `defval`::
+ *   default value to fill (*->value) with for PARSE_OPT_OPTARG.
+ *   CALLBACKS can use it like they want.
+ */
 struct option {
 	enum parse_opt_type type;
 	int short_name;
@@ -32,8 +67,6 @@ struct option {
 
 	int flags;
 	parse_opt_cb *callback;
-	/* holds default value for PARSE_OPT_OPTARG,
-	   though callbacks can use it like they want */
 	intptr_t defval;
 };