Clarify the acceptable values for `OPAMVERBOSE`

[smorimoto]

As far as I know, they are not documented at the moment. Actually, if you follow the implementation, you will get here: https://github.com/ocaml/opam/blob/bb1400b92a7971cfd413aeb1c56076ac83b5b016/src/core/opamStd.ml#L1670-L1674

[rjbou]

There is documentaiton in the manpage, that is a redirection ftm:

OPAMVERBOSE see option `--verbose'.

leading to

-v, --verbose
  Be  more verbose. One -v shows all package commands, repeat to also display commands 
  called internally (e.g. tar, curl, patch, etc.) Repeating n times is equivalent to 
  setting $OPAMVERBOSE to "n".

The used function is https://github.com/ocaml/opam/blob/44d7bd9d0f0606a1e939f953c22980772c44430f/src/core/opamStd.ml#L1682-L1689

We could add something like "takes an integer from 0, for more details, see --verbose", in order to precise what is the type of the input, and not introduce repetition.