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:

opam/src/core/opamStd.ml

Lines 1670 to 1674 in bb1400b

	let bool_of_string s =
	match String.lowercase_ascii s with
	| "" | "0" | "no" | "false" -> Some false
	| "1" | "yes" | "true" -> Some true
	| _ -> None

[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

opam/src/core/opamStd.ml

Lines 1682 to 1689 in 44d7bd9

	let env_level var =
	env (function s ->
	if s = "" then 0 else
	match bool_of_string s with
	| Some true -> 0
	| Some false -> 1
	| None -> int_of_string s)
	var

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.