Fix std::fputws return value documentation

[Radfordhound]

The current Microsoft documentation for std::fputws incorrectly states that it "returns WEOF", when, in reality, it returns EOF as-per the C/C++ standard.

I've verified this by checking ucrt's "stdio\fputws.cpp", which returns EOF and also makes a note of this in a comment:

(Note well that we return EOF and not WEOF on failure.  This is intentional, and is the correct behavior per the C Language Standard).

[PRMerger8]

@Radfordhound : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[TylerMSFT]

Thank you for improving the docs!