Improve API docs on ErrorDefinitions

[funkyfuture]

Used Cerberus version / latest commit: any

• I have the capacity to improve the docs when my problem is solved.

• I have the capacity to submit a patch when a bug is identified.

• My question does not concern a practical use-case that I can't figure out
  to solve.

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Bug report

the api docs exclude error definitions that are not assigned to a distinct rule. that's quark.

on that occasion, the semantics of the codes' bits should be documented better and bits that are safe to use for customizations should be declared (mind different notations!). ther might be an open issue related to the latter.