Add deprecation advice to developer guide.

Perhaps this is excessively picky, but my understanding (and dictionary.com's) of the word 'brief' is "not lengthy in time" rather than "not lengthy in space". As such, whenever I read this sentence I get a mental image of the warning flashing up on the screen then fading away again. You could replace it with 'short', which does more have the meaning of "not lengthy in space".