[docs] add -ffp-cast-overflow-workaround to the release notes

This option was added with:
D46135
rL331041
...copying the text from UsersManual.rst for more exposure.

llvm-svn: 331056

clang/docs/ReleaseNotes.rst

@@ -83,6 +83,15 @@ Non-comprehensive list of changes in this release
 New Compiler Flags
 ------------------
 
+- :option:`-ffp-cast-overflow-workaround` and
+  :option:`-fnofp-cast-overflow-workaround`
+  enable (disable) a workaround for code that casts floating-point values to
+  integers and back to floating-point. If the floating-point value is not
+  representable in the intermediate integer type, the code is incorrect
+  according to the language standard. This flag will attempt to generate code
+  as if the result of an overflowing conversion matches the overflowing behavior
+  of a target's native float-to-int conversion instructions.
+
 - ...
 
 Deprecated Compiler Flags