+ update math docs

Author: skirpichev

Doc/library/math.rst

@@ -874,7 +874,7 @@ Constants
 
    The :mod:`math` module consists mostly of thin wrappers around the platform C
    math library functions.  Behavior in exceptional cases follows Annex F of
-   the C99 standard where appropriate.  The current implementation will raise
+   the C99 standard, if :attr:`sys.float_info.stdc_iec_559` is enabled.  The current implementation will raise
    :exc:`ValueError` for invalid operations like ``sqrt(-1.0)`` or ``log(0.0)``
    (where C99 Annex F recommends signaling invalid operation or divide-by-zero),
    and :exc:`OverflowError` for results that overflow (for example,