mailmap doc: start by mentioning the comment syntax

Mentioning the comment syntax and blank line support first is in line
with how "git help config" describes its format. See
b8936cf060 (config.txt grammar, typo, and asciidoc fixes, 2006-06-08)
for the paragraph I'm copying & amending from its documentation.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitmailmap.txt

@@ -23,6 +23,9 @@ canonical real names and email addresses.
 SYNTAX
 ------
 
+The '#' character begins a comment to the end of line, blank lines
+are ignored.
+
 In the simple form, each line in the file consists of the canonical
 real name of an author, whitespace, and an email address used in the
 commit (enclosed by '<' and '>') to map to the name. For example:
@@ -94,8 +97,6 @@ Other Author <other@author.xx>         <nick2@company.xx>
 Santa Claus <santa.claus@northpole.xx> <me@company.xx>
 ------------
 
-Use hash '#' for comments that are either on their own line, or after
-the email address.
 
 
 SEE ALSO