Update the documentation for Covariance.
Update the documentation for Covariance.

Remove some of the dire warnings about instability
as the implementation is reasonably stable.

Change-Id: I3b64cab04e4cda54c671fcf8a2ca5d95c15037bf

kartikkumar/ceres-solver,jmwalls/ceres-solver-fork,DougFirErickson/ceres-solver,jmwalls/ceres-solver-fork,jmwalls/ceres-solver-fork,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,timfel/ceres-bin,kartikkumar/ceres-solver,entylop/ceres-solver,stonier/ceres_solver,ikoruk/ceres-solver,ikoruk/ceres-solver,whuaegeanse/ceres-solver,entylop/ceres-solver,ysonggit/ceres-solver,timfel/ceres-bin,ysonggit/ceres-solver,stonier/ceres_solver,whuaegeanse/ceres-solver,DougFirErickson/ceres-solver,jmwalls/ceres-solver-fork,ysonggit/ceres-solver,kartikkumar/ceres-solver,entylop/ceres-solver,ikoruk/ceres-solver,timfel/ceres-bin,ikoruk/ceres-solver,stonier/ceres_solver,whuaegeanse/ceres-solver,kartikkumar/ceres-solver,timfel/ceres-bin,whuaegeanse/ceres-solver,stonier/ceres_solver,entylop/ceres-solver,ysonggit/ceres-solver,ikoruk/ceres-solver