[docs] Fix documentation of -fno-strict-float-cast-overflow after D115804.

[craig.topper]

Summary

Previously this was documented as having the behavior of the
"target's native float-to-int conversion". After D115804, clang
uses saturating FP cast intrinsics which have the same behavior
on all targets.

[nikic]

LGTM

clang/docs/ReleaseNotes.rst

- The default setting of FP contraction (FMA) is now -ffp-contract=on (for
   was not set).
   Related to this, the switch -ffp-model=precise now implies -ffp-contract=on
   rather than -ffp-contract=fast, and the documentation of these features has
   been clarified. Previously, the documentation claimed that -ffp-model=precise
   was the default, but this was incorrect because the precise model implied
   -ffp-contract=fast, whereas the (now corrected) default behavior is
   -ffp-contract=on.
   -ffp-model=precise is now exactly the default mode of the compiler.
+- -fstrict-float-cast-overflow no longer has target specific behavior. Clang
+  will saturate towards the smallest and largest representable integer values.
+  NaNs will be converted to zero.
 
 Internal API Changes
 --------------------
 
 - A new sugar ``Type`` AST node represents types accessed via a C++ using
   declaration. Given code ``using std::error_code; error_code x;``, ``x`` has
   a ``UsingType`` which desugars to the previous ``RecordType``.
 

clang/docs/UsersManual.rst

 
 .. _opt_fstrict-float-cast-overflow:
 
 **-f[no-]strict-float-cast-overflow**
 
    When a floating-point value is not representable in a destination integer
    type, the code has undefined behavior according to the language standard.
    By default, Clang will not guarantee any particular result in that case.
-   With the 'no-strict' option, Clang attempts to match the overflowing behavior
-   of the target's native float-to-int conversion instructions.
+   With the 'no-strict' option, Clang will saturate towards the smallest and
+   largest representable integer values instead. NaNs will be converted to zero.
 
 .. _opt_fmath-errno:
 
 **-f[no-]math-errno**
 
    Require math functions to indicate errors by setting errno.
    The default varies by ToolChain.  ``-fno-math-errno`` allows optimizations
    that might cause standard C math functions to not set ``errno``.