bug#20180: Missing documentation about redisplay.

[Alan Mackenzie]

Hello, Emacs.

The elisp manual doesn't contain adequate documentation about
(re)display.

The topics not covered include
1. What triggers redisplay.
2. What portions of the frame/all frames get redisplayed when a
  redisplay occurs, and how does this relate to the answer to 1..

There is a partial answer to 1. on the page "Forcing Redisplay" which
states that "Emacs normally tries to redisplay the screen whenever it
waits for input.", but this is clearly incomplete - redisplay also
happens in the absence of input (e.g. by context fontification).

One clearly needs an answer to 2. if one ever wants to cause the
redisplay of a particular part of a window or frame.

-- 
Alan Mackenzie (Nuremberg, Germany).