fix error message of option as with invalid charters in partial rendering

before this PR IDENTIFIER_ERROR_MESSAGE could lead to misunderstand the convention of partial name.
Added OPTION_AS_ERROR_MESSAGE for unvalid charter in as option.
Grammar pass through Integration Testing section [ci skip]
Move longer form example from integration guide to api reference, below an
existing example. [ci skip]

My reasoning is that this is probably too much information for the complete
Rails testing guide, as we're trying to cover testing all aspects of the
framework.