StyleCopAnalyzers: Add option to ignore documentation errors on internal members and types

The documentation checkers are really useful for ensure consistency and completeness of XML doc for later generating API reference documentation. However if you put a <summary> tag on an internal member for your own sake, but don’t follow all the rules for documenting the entire member as the rules dictate, you get a lot of warnings.

Most of the warnings aren’t really that important for internal documentation, as most of it pertain to generating API doc. I’d like to be able to turn off the documentation checks for anything not public. Or a general pattern to set the global suppression of a rule to only apply to public/internal/etc. (as long as we all promise not to misuse them 😄 )