Add documentation to attributes in Q#

[orpuente-MS]

Is your feature request related to a problem? Please describe.
We have added a couple more attributes since the last release and we plan to add a few more going forward. It would be helpful to the users to have some documentation show up when you hover over an attribute.

Describe the solution you'd like
The documentation strings can be hard coded into the language service, since there is no way to define custom attributes in Q#.

A screenshot showing the lack of documentation when you hover over an attribute.