Documentation: alias: add notes on shell expansion

When writing inline shell for shell-expansion aliases (i.e. prefixed
with "!"), there are some caveats around argument parsing to be aware
of.  This series of notes attempts to explain what is happening more
clearly.

Signed-off-by: Ian Wienand <iwienand@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/alias.txt

@@ -27,3 +27,17 @@ it will be treated as a shell command.  For example, defining
   repository, which may not necessarily be the current directory.
 * `GIT_PREFIX` is set as returned by running `git rev-parse --show-prefix`
   from the original current directory. See linkgit:git-rev-parse[1].
+* Shell command aliases always receive any extra arguments provided to
+  the Git command-line as positional arguments.
+** Care should be taken if your shell alias is a "one-liner" script
+   with multiple commands (e.g. in a pipeline), references multiple
+   arguments, or is otherwise not able to handle positional arguments
+   added at the end.  For example: `alias.cmd = "!echo $1 | grep $2"`
+   called as `git cmd 1 2` will be executed as 'echo $1 | grep $2
+   1 2', which is not what you want.
+** A convenient way to deal with this is to write your script
+   operations in an inline function that is then called with any
+   arguments from the command-line.  For example `alias.cmd = "!c() {
+   echo $1 | grep $2 ; }; c" will correctly execute the prior example.
+** Setting `GIT_TRACE=1` can help you debug the command being run for
+   your alias.