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

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>