Missing docs for S3 methods?

[hadley]

I think it would be useful to document mutate(), filter() etc methods so its obvious that duckplyr supports them. This would also be a good place to document any differences in behaviour.

[krlmlr]

DoD:

• Understand and perhaps avoid collisions that occur for ?mutate as soon as duckplyr is loaded
• Draft for 2-3 verbs, reviewed
• Documentation for verbs similarly to ?dbplyr::mutate.tbl_lazy
• Fallback conditions are documented
• Most important differences in behavior are documented
• Add list of verbs that are not implemented