bpo-46468: document that "-m http.server" defaults to port 8000 (GH-30776)

Code link:
https://github.com/python/cpython/blob/70c16468deee9390e34322d32fda57df6e0f46bb/Lib/http/server.py#L1270

It's been this way since at least 3.4.

Also improved some wording in the same section.