NH-2444: Adding some documentation for Linq querying API.

[fredericDelaporte]

NH-2444 ask for documenting the Linq provider. Here is a minimal documentation for it.
There is no "unsupported feature" section as asked in a comment: this may change too frequently in my opinion. (And I do not have an exhaustive knowledge of it.)
There is no "extensions" section too: I feel it a bit hard to properly document.

Any feedback welcome!

[fredericDelaporte]

I think I should add a "supported data types methods and properties" section for documenting things like support of DateTime.Year, string.Length, and whatever else I find in the DefaultLinqToHqlGeneratorsRegistry.
I may move there a part of the "miscellaneous" section, which may then disappear (I do not like such section).
And by the way I could give some starting point for adding custom generators, but without much more explanations than "go see the sources for examples", especially about expression conversion to HQL trees.

[fredericDelaporte]

"Supported methods and members" section committed. "Miscellaneous" is shorter but still there, and I have not added explanations for adding custom generator for now.

[fredericDelaporte]

Minimal explanations for a custom generator added. Heavily example based, and far from covering all cases, it recommends checking NHibernate sources for more. (Should I include a direct link on Github?)
There is no "misc" section anymore.

I do not think I will made more changes, unless I have to take into account some feedback.