Rework alias and public room list rules docs

[DMRobertson]

This at least makes it easier for admins to deal with situations like those described in #15722.

Preview: https://pr16541--synapse-docs-previews.netlify.app/usage/configuration/config_documentation#alias_creation_rules

cc @xaur —please let me know if you think this wording is an improvement.

[xaur]

@DMRobertson this wording is a huge improvement. Appreciate explicit statements about the defaults (permit all) and that an empty list denies all. Also, I didn't know that alias_creation_rules: is not an empty list without []. Good to have more examples, too.

[DMRobertson]

Also, I didn't know that alias_creation_rules: is not an empty list without [].

For completeness: the yaml document as a whole is parsed as a mapping (python: dictionary). So alias_creation_rules: ... is a key-value pair in this map. The value in this case is a YAML null (python: None)

>>> import yaml
>>> yaml.safe_load("alias_creation_rules:")
{'alias_creation_rules': None}```