Rollup merge of #68627 - joshtriplett:write-all-none, r=Dylan-DPC

Document that write_all will not call write if given an empty buffer

Some types of Write instances have a semantic meaning associated with
writing an empty buffer, such as sending an empty packet. This works
when calling `write` directly, and supplying an empty buffer. However,
calling `write_all` on an empty buffer will simply never call `write`,
because `write_all` assumes it has no work to do.

Document this behavior, to help prospective users of
datagram-packet-style Write instances.

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