[NFC] Adding a note about the macro __FLT_EVAL_METHOD__

[zahiraam]

Summary

This is to clarify that the macro FLT_EVAL_METHOD it is not pre-defined like other preprocessor macros. It will not appear when preprocessor macros are dumped.

[aaron.ballman]

clang/docs/UsersManual.rst

1609–1619

I reworked this a bit more -- does it still seem correct to you?

[zahiraam]

clang/docs/UsersManual.rst

1609–1619

Yes. Thanks (long sentences :).

[aaron.ballman]

LGTM, thanks for the cleanup here!

clang/docs/UsersManual.rst

.. option:: -f[no-]protect-parens:
    when `-fprotect-parens` is enabled.  Also note that in safe floating-point
    modes, such as `-ffp-model=precise` or `-ffp-model=strict`, this option
    has no effect because the optimizer is prohibited from making unsafe
    transformations.
 
 .. _FLT_EVAL_METHOD:
 
 A note about ``__FLT_EVAL_METHOD__``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-The macro ``__FLT_EVAL_METHOD__`` will expand to either the value set from the
-command line option ``ffp-eval-method`` or to the value from the target info
-setting. The ``__FLT_EVAL_METHOD__`` macro cannot expand to the correct
-evaluation method in the presence of a ``#pragma`` which alters the evaluation
-method. An error is issued if ``__FLT_EVAL_METHOD__`` is expanded inside a scope
-modified by ``#pragma clang fp eval_method``.
+The ``__FLT_EVAL_METHOD__`` is not defined as a traditional macro, and so it
+will not appear when dumping preprocessor macros. Instead, the value
+``__FLT_EVAL_METHOD__`` expands to is determined at the point of expansion
+either from the value set by the ``-ffp-eval-method`` command line option or
+from the target. This is because the ``__FLT_EVAL_METHOD__`` macro
+cannot expand to the correct evaluation method in the presence of a ``#pragma``
+which alters the evaluation method. An error is issued if
+``__FLT_EVAL_METHOD__`` is expanded inside a scope modified by
+``#pragma clang fp eval_method``.
 
 .. _fp-constant-eval:
 
 A note about Floating Point Constant Evaluation
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 In C, the only place floating point operations are guaranteed to be evaluated
 during translation is in the initializers of variables of static storage
 duration, which are all notionally initialized before the program begins