Allow frame instances to be passed to SkyCoord
Updated documentation
Fix issue when frame instance was passed as positional argument
Check that data from frame gets ignored
Don’t allow frames with data
Implement new behavior based on discussion
Fixed comments
Clarify use of SkyCoord as frame in docs
Merge pull request #3063 from astrofrog/sky-coord-frame-instance

Allow frame instances to be passed to SkyCoord