Clarify behavior of GetWindowTextLength towards zero-length titles

[sylveon]

I've recently hit that undocumented case myself, so I thought it would be great to add them to the docs to make the behavior of the function less confusing.

[PRMerger9]

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

[Karl-Bridge-Microsoft]

thanks @sylveon

[Karl-Bridge-Microsoft]

#sign-off

[sylveon]

This function does not clear the most recent error information. To determine success or failure, clear the most recent error information by calling SetLastError with 0, then call GetLastError.

This might be a bit confusing, I think that it should be pointed out that SetLastError must be called before GetWindowTextLength and that GetLastError should called after.