git-am: Clean up the asciidoc documentation

git-am: Clean up the asciidoc documentation

Add --keep to synopsis.

The synopsys used a mix of tabs and spaces, unify to use only
spaces.

Shuffle options around in synopsys and description for grouping
them logically.

Add more gitlink references to other commands.

Various grammatical fixes and improvements.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>