Improve preconditioner documentation.
Improve preconditioner documentation.

Change-Id: Ied811fc16f0c1c553e37af6d3111de71baea8462

whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,ysonggit/ceres-solver,whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,DougFirErickson/ceres-solver,ysonggit/ceres-solver,ysonggit/ceres-solver,ysonggit/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver