Documentation: fix typos, grammar, asciidoc syntax

[jc: the original patch was against master but 99% of it
 applied to maint; this commit splits out the part that
 applies only to master.]

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-diff-tree.txt

@@ -63,7 +63,7 @@ and terminated by a newline) is printed before the difference.  When
 comparing commits, the ID of the first (or only) commit, followed by a
 newline, is printed.
 +
-The following flags further affects the behavior when comparing
+The following flags further affect the behavior when comparing
 commits (but not trees).
 
 -m::