mailinfo documentation: accurately describe non -k case

mailinfo documentation: accurately describe non -k case

Since its very first description of -k, the documentation for
git-mailinfo claimed that (in the case without -k) after cleaning up
bracketed strings [blah], it would insert [PATCH].

It doesn't; on the contrary, one of the important jobs of mailinfo is
to remove those strings.

Since we're already there, rewrite the paragraph to give a complete
enumeration of all the transformations.  Specifically, it was missing
the whitespace normalization (run of isspace(c) -> ' ') and the
removal of leading ':'.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>