document bashs globstar setting

runtime/doc/editing.txt

@@ -385,7 +385,9 @@ as a wildcard when "[" is in the 'isfname' option.  A simple way to avoid this
 is to use "path\[[]abc]", this matches the file "path\[abc]".
 
 					*starstar-wildcard*
-Expanding "**" is possible on Unix, Win32, macOS and a few other systems.
+Expanding "**" is possible on Unix, Win32, macOS and a few other systems (but it
+may depend on your shell setting. It's known to work correctly for zsh, for
+bash you need to configure it specifically |bash-starstar-wildcard|)
 This allows searching a directory tree.  This goes up to 100 directories deep.
 Note there are some commands where this works slightly differently, see
 |file-searching|.
@@ -407,6 +409,18 @@ Note that the path with "/sys" is included because it does not need to match
 "/inc".  Thus it's like matching "/usr/inc*/*/*...", not
 "/usr/inc*/inc*/inc*".
 
+					*bash-starstar-wildcard*
+Bash supports expansion of the "**" pattern only, when the globstar setting
+is enabled in your bash configuration file (typically .bashrc).  But this
+requires at least bash version 4. To make filename expansion in Vim using the
+"**" pattern work when you are using the bash shell, add the following setting
+to your .bashrc file: >
+	shopt -s globstar
+Now that bash is prepared to expand the ** pattern, we need to tell Vim to use
+and interactive shell, whenever it needs to expand the ** pattern, so that it
+will source your bashrc file. To do set, set the following: >
+	set shellcmdflag=-ic
+<
 					*backtick-expansion* *`-expansion*
 On Unix and a few other systems you can also use backticks for the file name
 argument, for example: >