Document opening diff split from patch

References #2219.

There's awesome special behaviour for buffers with diff output to view
it in a split diff instead of unified diff.

doc/fugitive.txt

@@ -357,6 +357,8 @@ Navigation maps ~
                         removed if a count was given.  If the line is still in
                         the work tree version, passing a count takes you to
                         it.
+                        On the 'diff --git' line of a patch, this and similar
+                        maps open a |:Gdiffsplit|.
 
                                                 *fugitive_o*
 o                       Open the file or |fugitive-object| under the cursor in