Emit C# XML Documentation Comments for "generated" members

[jonpryor]

There are various scenarios where generator will "create" types and members which are related to Java types, but not a direct binding. Because these scenarios are not directly related to a Java member, there is no Javadoc to import for them, and thus they are never documented.

These scenarios include:

• "Infrastructure" such as the JniPeerMembers property
• async overloads: https://github.com/xamarin/monodroid/commit/f99795593de64e21e7919b32bfa06843ab42b89e http://github.com/xamarin/monodroid/commit/698a54180cc26909adb40f93a8255e368790841e
  C# XML doc comments could link to the "source" method, copy over parameter docs, etc.
• EventArgs subclasses; docs could link to the relevant interface method
• EventArgs subclass members; docs could mention the parameter name they belong to, possibly copy the parameter name docs?
• Enum types?
• Enum members; docs could copy the "source" constant documentation

[pjcollins]

Adding the javadoc tag as this is doc generation related, even though it's not specific to Javadoc importing.