Include default values in API docs

I'm encountering some API docs where the default is not obvious. The two I encountered were `File.GetFiles()` `EnumerationOptions`. Neither `MatchCasing` nor `MatchType` seem to indicate a default.

This may be a deeper problem in the structure of XML docs, which may make it difficult to gather this information. It seems we should work on this, such as by a pattern of including in API docs " Default." at the end of each default, or by recognizing parameter defaults if they are used, or something else.

This is needed because as far as I can tell, there is no way to get this information except running the code. Well, should we trust the zero value of an enum to always be the default?

(and if it is there and I am missing it, please review visibility)