completion: loosen and document the requirement around completing alias

completion: loosen and document the requirement around completing alias

Recently we started to tell users to spell ": git foo ;" with
space(s) around 'foo' for an alias to be completed similarly
to the 'git foo' command.  It however is easy to also allow users to
spell it in a more natural way with the semicolon attached to 'foo',
i.e. ": git foo;".  Also, add a comment to note that 'git' is optional
and writing ": foo;" would complete the alias just fine.

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