Clean up XML documentation comments

[stakx]

There have been some inaccuracies in the XML documentation comments for a long while, it would be nice to clean them up. This issue is a list to track those before they're forgotten again. Feel free to add anything to it that you find.

• Mocks class in src/Moq/Linq/Mocks.cs:

  https://github.com/moq/moq4/blob/bb4142271cf67c9da2ddfc6e5b7395d529f3240e/src/Moq/Linq/Mocks.cs#L23-L25

  This doesn't appear to be true any more, or at least it isn't clear which extension methods on object this refers to. (Perhaps more features were in the planning at some point in the past than we have now?)

[stakx]

• Typo in several LINQ to mocks methods: an mock → a mock:

  https://github.com/moq/moq4/blob/8eb41b38531b372a55c815b105bb4bd010d90e96/src/Moq/Linq/Mocks.cs#L44-L51

• Also, several LINQ methods mention a specification although the method(s) do not have an expression / specification parameter (or vice versa):

  https://github.com/moq/moq4/blob/8eb41b38531b372a55c815b105bb4bd010d90e96/src/Moq/Linq/Mocks.cs#L22-L27

[stakx]

• The XML documentation for Mock<T>'s T type parameter states that: "Type to mock, which can be an interface or a class." which is incomplete. T can also be a delegate.