Improve documentation of admin/domain_allows

[ThisIsMissEm]

Currently the admin/domain_allows API & overall feature seems to be lacking documentation, which could lead to it not being correctly understood. It looks like this feature is only used if LIMITED_FEDERATION_MODE (formerly WHITELIST_MODE) is enabled.

If LIMITED_FEDERATION_MODE is enabled, then the Domain Blocks feature is ignored, and the Domain Allows feature is used instead. This behaviour should be documented on the entry for setting: https://docs.joinmastodon.org/admin/config/#limited_federation_mode

Additionally, we should document that if you wish to change from true to false, then you should first import a blocklist from one of the many providers, as to avoid exposing your users to harm.

The documentation at https://docs.joinmastodon.org/methods/admin/domain_allows/ should read something like this at the top:

This API creates rules to allow only the specified domains to federate with your instance. This only applies when [LIMITED_FEDERATION_MODE] is enabled. This is functionality is the inverse of the [Domain Blocks] functionality, and cannot be used to override a wild-card domain block.

Additionally, the Domain Blocks API should probably have documentation stating the inverse of the above.