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>