Add doc where MutableCharSpan is used to set it's size to 0 if not used.

[hicklin]

Following this comment, add documentation wherever a referance to a MutableCharSpan is passed to say that if this is not edited successfully, it's size should be set to 0.

example, after "@param[out] statusText text describing failure (see description above), size kMaxSizeStatusText."

"If writing to statusText can result in a failure, example due to a string that can be longer that kMaxSizeStatusText, the size of this value should be set to 0 with .reduce_size(0)."