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

打开 Doc/library/imaplib.rst 并找到 IMAP4.append() 的公开文档。更新该文档，使其涵盖消息必须为 bytes-like 类型的要求、哪些参数可以为 None、falsy 的邮箱值默认使用 INBOX，以及 date_time=None 会省略内部日期参数；关联的 PR 表明这项工作已经在进行中。