Verification of Tokens by getRequiredToken method

triggered by https://github.com/checkstyle/checkstyle/issues/2388 .

We need to decided what to do when user specify in "tokens" not all tokens that are in requiredTokens list.

Possible approaches:
1) Checkstyle should give an error to user (fail-fast) when he try to put in "tokens" incomplete set of tokens according to getRequiredTokens.

2) The original intention of tokens was rather to specify additional tokens, not all. So requiredTokens are always appended automatically.

That issue is also related to issue of describing to user getRequiredTokens . Issue to describe getAvailableTokens and getDefaultTokens in XDOC.
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