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.

Author: tbwolfe

Doc/library/sys.rst

@@ -604,12 +604,18 @@ always available.
    +---------------------+----------------+--------------------------------------------------+
    | :const:`radix`      | FLT_RADIX      | radix of exponent representation                 |
    +---------------------+----------------+--------------------------------------------------+
-   | :const:`rounds`     | FLT_ROUNDS     | integer constant representing the rounding mode  |
-   |                     |                | used for arithmetic operations.  This reflects   |
-   |                     |                | the value of the system FLT_ROUNDS macro at      |
-   |                     |                | interpreter startup time.  See section 5.2.4.2.2 |
-   |                     |                | of the C99 standard for an explanation of the    |
-   |                     |                | possible values and their meanings.              |
+   | :const:`rounds`     | FLT_ROUNDS     | integer representing the rounding mode for       |
+   |                     |                | floating-point arithmetic. This reflects the     |
+   |                     |                | value of the system FLT_ROUNDS macro at          |
+   |                     |                | interpreter startup time:                        |
+   |                     |                | ``-1`` indeterminable,                           |
+   |                     |                | ``0`` toward zero,                               |
+   |                     |                | ``1`` to nearest,                                |
+   |                     |                | ``2`` toward positive infinity,                  |
+   |                     |                | ``3`` toward negative infinity                   |
+   |                     |                |                                                  |
+   |                     |                | All other values for FLT_ROUNDS characterize     |
+   |                     |                | implementation-defined rounding behavior.        |
    +---------------------+----------------+--------------------------------------------------+
 
    The attribute :attr:`sys.float_info.dig` needs further explanation.  If