gh-85988: Change documentation for sys.float_info.rounds (GH-99675)

* Change documentation for sys.float_info.rounds

Change the documentation for sys.float_info.rounds to remove
references to C99 section 5.2.4.2.2 and instead place the
available values inline.

* Correction to previous documentation change

Newlines were not preserved in generated HTML on previous
commit. I have changes the list to a comma-separated list
of values and their meanings.

* Clarify source for value of FLT_ROUNDS

Clarify the source of the FLT_ROUNDS value and
change 'floating-point addition' to 'floating-point
arithmetic' to indicate that the rounding mode
applies to all arithmetic operations.