lib/cli: add comment clarifying behavior

It wasn't clear to me why `toGNUCommandLine` would take an input like
this `foo = "bar"`, and turn it into `[ "foo" "bar" ]`. I implemented
a change to this, only to realize that this behavior is actually for
`toGNUCommandLineShell`. It uses `lib.escapeShellArgs`, which treats
the input `[ "--foo" "bar" ]` and `[ "--foo bar" ]` differently. The
comment will hopefully prevent others from going down the same rabbit
hole that I did.

lib/cli.nix

@@ -137,6 +137,7 @@ rec {
         in
         if v == null then
           [ ]
+        # We need the option and its value in different list elements, to play nice with `lib.escapeShellArgs`
         else if optionValueSeparator == null then
           [
             (mkOptionName k)