The function name "operator poll message set" is rather troublesome, as:
- the message is only used when the operator is disabled, and
- the message is shown if the operator is disabled by any means, and not
just limited to the `poll()` function returning `false`.
A better name would be `CTX_wm_operator_disabled_msg_set`, but refactoring
that is for another time. Now at least the behaviour is documented.
No functional changes.
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