doc/diff-options: fix link to generating patch section

When formatted as man-page, the section title is rendered
"GENERATING PATCH TEXT WITH -P" whereas reference still reads
"Generating patch text with -p", that is inconsistent and makes
searching harder than it needs to be.

Fix this by getting rid of custom reference text.

Also, documentation for every command that describes `-p` option by
including the "diff-options.txt" file does include the
"diff-generate-patch.txt" file as well (as it should), so the internal
link is in fact useful for any of them.

Fix this by getting rid of conditionals around the reference.

Fixes: ebdc46c242 (docs: link generating patch sections)
Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -22,13 +22,7 @@ ifndef::git-format-patch[]
 -p::
 -u::
 --patch::
-	Generate patch (see section titled
-ifdef::git-log[]
-<<generate_patch_text_with_p, "Generating patch text with -p">>).
-endif::git-log[]
-ifndef::git-log[]
-"Generating patch text with -p").
-endif::git-log[]
+	Generate patch (see <<generate_patch_text_with_p>>).
 ifdef::git-diff[]
 	This is the default.
 endif::git-diff[]