docs: document retry_if_exception_message message vs match semantics

[HUIIIM]

— vertciti AI team (assisting Jiahui Miao)

Summary

retry_if_exception_message and retry_if_not_exception_message are currently
listed only as bare items under "See also these methods" in the "Whether to
retry" section, with no examples and no description of their semantics [实证].
This PR documents them.

What this documents

• retry_if_exception_message(message=...) retries when
  message == str(exception) — an exact-equality check against the
  exception's string form [实证].
• retry_if_exception_message(match=...) retries when
  re.compile(match).match(str(exception)) matches. Note this is re.match
  semantics — anchored at the start of the string, not re.search [实证].
  So match="timeout" does not match "Connection timeout"; use
  match=r".*timeout.*" for substring semantics.
• Exactly one of message / match must be passed; passing neither raises
  TypeError [实证].
• Since PR Accept empty message in retry_if_exception_message #651 (commit 40fad186bae14796567ea6786d8701bdd9d79312),
  an empty string "" is accepted as a message target [实证].
• retry_if_not_exception_message is the logical negation of the above [推断].

Labels: [实证] = verified from the tenacity source; [推断] = inference.

Testing

Docs-only change. [未确认: docs build not run in this pass — suggest CI
preview before merge.]