guix: Add clarifying documentation for V env var

contrib/guix/README.md

@@ -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