DOC: Whatsnew for string coercion fix [ci skip]
xref https://github.com/pandas-dev/pandas/pull/21366/files/a94d3995160a07a09693a883ceb0641e82ca7bdd#r194103331

That PR had
tests (https://github.com/pandas-dev/pandas/pull/21366/files/a94d3995160a07a09693a883ceb0641e82ca7bdd#diff-10e7ab03eb0363417c9b042860d3ba73R814)
but no release note for this specifically, since I didn't know it was a bug.

[ci skip]