Recommit -r372180

Commit message below, original caused the sphinx build bot to fail, this
one should fix it.

Create UsersManual section entitled 'Controlling Floating Point
Behavior'

Create a new section for documenting the floating point options. Move
all the floating point options into this section, and add new entries
for the floating point options that exist but weren't previously
described in the UsersManual.

Patch By: mibintc
Differential Revision: https://reviews.llvm.org/D67517

llvm-svn: 372229