Document flymake fringe bitmaps

Document flymake fringe bitmaps

* doc/misc/flymake.texi (Customizable variables)
(Highlighting erroneous lines): Mention flymake-error-bitmap,
flymake-warning-bitmap, and flymake-fringe-indicator-position.

* lisp/progmodes/flymake.el (flymake-error-bitmap)
(flymake-warning-bitmap, flymake-fringe-indicator-position): Doc fixes.
(flymake-error-bitmap, flymake-warning-bitmap): Fix :types.

* etc/NEWS: Related markup.