git-help.txt: document "git help cmd" vs "git cmd --help" for aliases

This documents the existing behaviour of "git help cmd" when cmd is an
alias, as well as providing a hint to use the "git cmd --help" form to
be taken directly to the man page for the aliased command.

Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-help.txt

@@ -29,6 +29,10 @@ guide is brought up. The 'man' program is used by default for this
 purpose, but this can be overridden by other options or configuration
 variables.
 
+If an alias is given, git shows the definition of the alias on
+standard output. To get the manual page for the aliased command, use
+`git COMMAND --help`.
+
 Note that `git --help ...` is identical to `git help ...` because the
 former is internally converted into the latter.