[#747] Add errorDocArg, deprecate errorDoc

Description

Problem: errorDoc hard-codes error argument to '()'. This is not always desirable and arguably not even a sensible default since the introduction of 'NoErrorArg' in #383 (closed).

Solution: To avoid introducing hard-to-catch backwards compatibility issues, define a new quasi-quoter, 'errorDocArg' that allows specifying the type of the error argument as an optional parameter. Define 'errorDoc' in terms of 'errorDocArg'. Deprecate 'errorDoc'.

Related issue(s)

Resolves #747 (closed)

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