chore(doc): doc for custom subcommands

src/doc/src/reference/external-tools.md

@@ -265,6 +265,12 @@ cargo `(?<command>[^ ]+)` into an invocation of an external tool
 `cargo-${command}`. The external tool must be present in one of the user's
 `$PATH` directories.
 
+> Note: Cargo looks for custom subcommands in `$PATH `and then `$CARGO_HOME/bin`, 
+> and uses the custom subcommands found in `$PATH` first. 
+> The custom subcommand is typically installed by default in `$CARGO_HOME/bin` 
+> and executed from there. However, if you prefer to invoke another subcommand elsewhere 
+> you can add it to the `$PATH` and keep it before `$CARGO_HOME/bin`. 
+
 When Cargo invokes a custom subcommand, the first argument to the subcommand
 will be the filename of the custom subcommand, as usual. The second argument
 will be the subcommand name itself. For example, the second argument would be