docs(imap-types): describe required capabilities in API

[duesee]

Closes #498

[coveralls]

Pull Request Test Coverage Report for Build 9724413705

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.678%

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Totals
Change from base Build 9658279125:	0.0%
Covered Lines:	10190
Relevant Lines:	10995

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💛 - Coveralls