tridentfb: documentation update

Make the tridentfb documentation closer to current state of the tridentfb
driver.  Fix also some formatting.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>