Changes

Fix some issues in the Map documentation:

  - Add the key argument in the description of 'merge'
  - Note that the keys of 'union f m1 m2' are a subset of the
    input keys, not all the keys, since

        bindings (union (fun _ _ _ -> None) m m) = []

  - Fix grammar in the descriptions of 'filter', 'union', 'merge'
  - Fix mismatched variable name in the description of 'partition'
  - Note that 'find' and 'find_opt' return values, not bindings
 (commit: afe681c)