message: guix: Add clarifying documentation for V env var
diff: @@ -142,6 +142,11 @@ find output/ -type f -print0 | sort -z | xargs -r0 sha256sum
  If non-empty, will pass `V=1` to all `make` invocations, making `make` output
  verbose.
  + Note that any given value is ignored. The variable is only checked for
  + emptiness. More concretely, this means that `V=` (setting `V` to the empty
  + string) is interpreted the same way as not setting `V` at all, and that `V=0`
  + has the same effect as `V=1`.
  +
  * _**ADDITIONAL_GUIX_ENVIRONMENT_FLAGS**_
  Additional flags to be passed to `guix environment`. For a fully-bootstrapped
  