doc: document '{a,b}' glob syntax

This syntax does not exist in `git`, so it is not documented in `man
gitignore`. There is a question of whether it *should* exist, but as
long as it does, it should be documented somewhere.

See also:
#1221
#1368

Closes #1816

CHANGELOG.md

@@ -45,6 +45,8 @@ Bug fixes:
   Clarify that CLI invocation must always be valid, regardless of config file.
 * [BUG #1741](https://github.com/BurntSushi/ripgrep/issues/1741):
   Fix stdin detection when using PowerShell in UNIX environments.
+* [BUG #1816](https://github.com/BurntSushi/ripgrep/issues/1816):
+  Add documentation for glob alternate syntax, e.g., `{a,b,..}`.
 * [BUG #1847](https://github.com/BurntSushi/ripgrep/issues/1847):
   Clarify how the `--hidden` flag works.
 * [BUG #1866](https://github.com/BurntSushi/ripgrep/issues/1866#issuecomment-841635553):

crates/core/app.rs

@@ -1358,6 +1358,13 @@ used. Globbing rules match .gitignore globs. Precede a glob with a ! to exclude
 it. If multiple globs match a file or directory, the glob given later in the
 command line takes precedence.
 
+As an extension, globs support specifying alternatives: *-g 'ab{c,d}'* is
+equivalet to '-g abc -g abd'. Empty alternatives like *-g 'ab{,c}'* are not
+currently supported. Note that this syntax extension is also currently enabled
+in gitignore files, even though this syntax isn't supported by git itself.
+ripgrep may disable this syntax extension in gitignore files, but it will
+always remain available via the -g/--glob flag.
+
 When this flag is set, every file and directory is applied to it to test for
 a match. So for example, if you only want to search in a particular directory
 'foo', then *-g foo* is incorrect because 'foo/bar' does not match the glob