Add comments to trust_region_minimizer.cc.
Add comments to trust_region_minimizer.cc.

trust_region_minimizer.cc now contains a comment that explains
the reasoning behind he inner iteration step acceptance change.

Change-Id: I4eaa69d6bab92c543bba3f119c09f44625d393bd

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