FAQ: add note on version ordering

closes #1651
Add 'opam show --where' to get the used opam file location

Useful to debug pinned packages; closes #1600
Add 'opam lint'

Closes #1602, although this may further be improved with warnings/errors, etc.
Merge pull request #1725 from OCamlPro/opam-lint

Add opam lint, opam show --where