Add a conclusion to "Data Types and Matching"

Explain slightly more what the exhaustiveness of pattern matching
means.  Fixes the first request of
https://github.com/ocaml/ocaml.org/pull/165/files#diff-7aa1248a16bc967c3c0d4a7a7caab077R271
Minor corrections

Inspired by
https://github.com/ocaml/ocaml.org/pull/165/files#diff-7aa1248a16bc967c3c0d4a7a7caab077R271