Expand and clarify "unkown" scope comment in git_cmd

- Mention `/Applications` as well as `/Library`, since macOS users
  with a full Xcode installation installation seem often to have
  Apple Git under `/Applications` rather than under `/Library`,
  but this higher-than-system "unknown" scope applies equally to
  both.

- Expand the explanation enough to make clear what the conceptual
  relationship is between `GIT_CONFIG_SYSTEM` and `--system` and
  why the disparity between the scopes they select could be
  misleading in a relevant way.

Author: EliahKagan

gix-path/src/env/git/mod.rs

@@ -136,8 +136,9 @@ fn git_cmd(executable: PathBuf) -> Command {
     //
     // Separately from that, we can't use --system here, because scopes treated higher than the
     // system scope are possible. This commonly happens on macOS with Apple Git, where the config
-    // file under `/Library` is shown as an "unknown" scope but takes precedence over the system
-    // scope. Although `GIT_CONFIG_NOSYSTEM` will suppress this as well, passing --system omits it.
+    // file under `/Library` or `/Applications` is shown as an "unknown" scope but takes precedence
+    // over the system scope. Although `GIT_CONFIG_NOSYSTEM` suppresses this scope along with the
+    // system scope, passing --system selects only the system scope and omit this "unknown" scope.
     cmd.args(["config", "-lz", "--show-origin", "--name-only"])
         .current_dir(cwd)
         .env_remove("GIT_CONFIG")