I withdraw my previously suggested addition to the Docstring glossary entry (msg281740).  It implies that trivial f-strings are acceptable and I agree that other implementations and future Cpython should be free to strictly follow the literal meaning of the first sentence: docstring = initial string literal expression.

I suggest instead that 'string literal' in the first sentence link to https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals.  This would make it clearer that 'string literal' is being used the the Python technical sense rather than in any more informal English sense.

We could possibly add a version of what Guido said above, such as: "(Acceptance of anything other than a string literal as a docstring is an implementation accident and should not be relied upon.)"