[clang][FPEnv] Clang floatng point model ffp-model=precise enables ffp-contract=on

This patch changes the ffp-model=precise to enables -ffp-contract=on
(previously -ffp-model=precise enabled -ffp-contract=fast). This is a
follow-up to Andy Kaylor's comments in the llvm-dev discussion
"Floating Point semantic modes". From the same email thread, I put
Andy's distillation of floating point options and floating point modes
into UsersManual.rst

Differential Revision: https://reviews.llvm.org/D74436