How official is __text_signature__?

• It is not described in the docs, only succinctly mentioned twice in the changelogs.
• The expected format of the docstring for a useful __text_signature__ to be generated is not documented.
• In particular, the docstring format described in PEP7 for PyDoc_STR(VAR) does not seem compatible with __text_signature__.

So: are custom C extensions expected to provide docstrings compatible with __text_signature__, and if so, what is the correct format?