docs: link generating patch sections

Currently, in the git-log documentation, the reference to generating
patches does not match the section title. This can make the section
"Generating patch text with -p" hard to find, since typically readers of
the documentation will copy and paste to search the page.

Let's make this more convenient for readers by linking it directly to
the section.

Since git-log pulls in diff-generate-patch.txt, we can provide a direct
link to the section. Otherwise, change the verbiage to match exactly
what the section title is, to at least make searching for it an easier
task.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-generate-patch.txt

@@ -1,3 +1,4 @@
+[[generate_patch_text_with_p]]
 Generating patch text with -p
 -----------------------------
 

Documentation/diff-options.txt

@@ -22,7 +22,13 @@ ifndef::git-format-patch[]
 -p::
 -u::
 --patch::
-	Generate patch (see section on generating patches).
+	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[]
 ifdef::git-diff[]
 	This is the default.
 endif::git-diff[]