Clarify encode_rfc2231 docstring: values are quoted even without charset or language

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: The encode_rfc2231 docstring states: "If neither charset nor language is given, then s is returned as-is."

Expected: The docstring promises 'a b' unchanged when charset and language are both None.

Actual: Returns 'a%20b'.

import email.utils

s = "a b"
if not s.isascii():
    print("REFUTATION REJECTED: input is not ASCII")
else:
    actual = email.utils.encode_rfc2231(s, charset=None, language=None)
    expected = s  # Documented return when neither argument is given.
    if actual != expected:
        print("REFUTATION CONFIRMED:", repr(s), repr(actual), repr(expected))
    else:
        print("REFUTATION REJECTED: actual matches documented expectation")

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library email.utils:

REFUTATION CONFIRMED: 'a b' 'a%20b' 'a b'

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/email.utils-20261003-021105-c4

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).