Document IMAP4.append() message type and optional arguments

The IMAP4.append() documentation currently says only:

Append message to named mailbox.

It now documents the flags argument, but the remaining argument behavior is still easier to discover from Lib/imaplib.py than from the library docs.

In the implementation:

• mailbox is treated as optional in practice: a false value is replaced with 'INBOX'.
• flags and date_time may be None, in which case they are omitted from the command.
• message is not optional. It is passed to MapCRLF.sub(CRLF, message), where MapCRLF is a bytes regular expression, so message must be a bytes-like object. Passing a str raises TypeError before the command is sent.

The method docstring already hints at this by saying "All args except 'message' can be None", but that detail is not in Doc/library/imaplib.rst.

This is related to the broader documentation issue in #68215, but this issue is intentionally narrower: clarify the public documentation for IMAP4.append() only.

Suggested documentation scope:

• State that message must be a bytes-like object.
• State which arguments may be None.
• Clarify that mailbox=None (or another false value) uses INBOX.
• Clarify that date_time=None omits the internal date argument.

This should stay separate from the existing CR/LF-normalization behavior tracked in #49680.

Linked PRs

• gh-149964

Apri Doc/library/imaplib.rst e individua la documentazione pubblica di IMAP4.append(). Aggiornala in modo da coprire il requisito che il messaggio sia di tipo bytes-like, quali argomenti possono essere None, il fatto che i valori falsy della mailbox usino INBOX come valore predefinito e che date_time=None ometta l’argomento della data interna; la PR collegata indica che questo lavoro è già in corso.