Add note about vertical alignment to patch guidelines.

Originally committed as revision 9695 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -1749,6 +1749,9 @@ do not attach several unrelated patches to the same mail.
 @item
     You should maintain alphabetical order in alphabetically ordered lists as
     long as doing so does not break API/ABI compatibility.
+@item
+    Lines with similar content should be aligned vertically when doing so
+    improves readability.
 @item
     Did you provide a suggestion for a clear commit log message?
 @end enumerate