bug#40784: Clarifying the difference between fringe bitmaps and XBM images

[Eli Zaretskii]

> Cc: 40784@debbugs.gnu.org
> From: Clément Pit-Claudel <cpitclaudel@gmail.com>
> Date: Thu, 23 Apr 2020 10:35:54 -0400
> 
> Of course: I am currently writing a mode that displays indicators either in 
> the margins or in the fringes, depending on the value of a defcustom.
> By default, I intended to use the same bitmaps in the margins and in the 
> fringes.  It took me a while to understand what I was doing wrong: I was 
> seeing reversed bitmaps, but I hadn't considered the possibility that the two 
> places where Emacs supports monochrome bitmaps would accept the same 
> representation (unibyte strings) but use a different bit order. 
> The proposed patch updates the documentation to save the next person from 
> experiencing the same pain.

If so, then why do we need to mention this in the doc string?  Won't
the manual be enough?  It is strange to mention just this factoid in
the doc strings, when one can shoot themselves in the foot with
bitmapped images in many exciting ways.