CancelRequest(F) documentation.

Hello,

CancelRequest(F) documentation at
https://www.postgresql.org/docs/18/protocol-message-formats.html#PROTOCOL-MESSAGE-FORMATS-CANCELREQUEST
seems to be inconsistent for protocol 3.2. It is stated here that the
length of the message is always 16 bytes (specified as Int32(16)),
however the maximum message length actually depends on the secret key
length (which can be up to 256 bytes, as also stated in the
documentation.)

I would propose to specify the overall message length as Int32 in the
documentation for PostgreSQL 18+.

Thanks.