Document memory stream clones for multiple workers

[nightcityblade]

NOTE Erasing or replacing the contents of this template will result in your pull
request being summarily closed without consideration!

Changes

Fixes #330.

Add a multiple-producer/multiple-consumer memory object stream example that gives each
task its own clone. The guide also explains why the original endpoints must be closed
and clarifies that receive clones distribute items rather than broadcast them.

Validation:

• pre-commit run -a
• pytest tests/streams/test_memory.py -q (136 passed)
• sphinx-build -n -E docs build/sphinx
• Executed the documented example and confirmed clean shutdown

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) which would fail without your patch
• You've updated the documentation (in docs/), in case of behavior changes or new
  features
• You've added a new changelog entry (in docs/versionhistory.rst).

Tests are not added because this PR only documents existing, already-tested behavior.

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #123, the entry should look like this:

- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

[agronholm]

LGTM!