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>

Documentation/git-mailinfo.txt

@@ -24,13 +24,24 @@ command directly.  See linkgit:git-am[1] instead.
 OPTIONS
 -------
 -k::
-	Usually the program 'cleans up' the Subject: header line
-	to extract the title line for the commit log message,
-	among which (1) remove 'Re:' or 're:', (2) leading
-	whitespaces, (3) '[' up to ']', typically '[PATCH]', and
-	then prepends "[PATCH] ".  This flag forbids this
-	munging, and is most useful when used to read back
-	'git format-patch -k' output.
+	Usually the program removes email cruft from the Subject:
+	header line to extract the title line for the commit log
+	message.  This option prevents this munging, and is most
+	useful when used to read back 'git format-patch -k' output.
++
+Specifically, the following are removed until none of them remain:
++
+--
+*	Leading and trailing whitespace.
+
+*	Leading `Re:`, `re:`, and `:`.
+
+*	Leading bracketed strings (between `[` and `]`, usually
+	`[PATCH]`).
+--
++
+Finally, runs of whitespace are normalized to a single ASCII space
+character.
 
 -b::
 	When -k is not in effect, all leading strings bracketed with '['