Document configuration option vocabulary/taxonomy and then simplify

This issue has been migrated from [#5584](https://github.com/matrix-org/synapse/issues/5584).

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A document that contains the vocabulary of the synapse configuration and the expected values of those named fields would assist in keeping code consistent.

A few things have popped up recently, here and in other branches, so I thought i'd start an issue to collect examples.

I think we should first generate a statement on what the best naming scheme is for things, aim to apply that in future, then come back and deprecate/rename existing options to bring them into line.

Related issues:
* https://github.com/matrix-org/synapse/issues/3913 (we should be clear this is used as a prefix or a template)
* https://github.com/matrix-org/synapse/issues/2326 (we should be clear on what is a template and what isn't)
* https://github.com/matrix-org/synapse/issues/3523 (we should be consistent in requiring and handling trailing slashes in URLs in our configuration)