docs: add global `--help` alias limitations

README.md

@@ -228,15 +228,17 @@ help() {
 
 Then you can do `$ help cp` or `$ help git commit`.
 
-Alternatively, you can also use global aliases to override `-h` and `--help` entirely:
+When you are using `zsh`, you can also use global aliases to override `-h` and `--help` entirely:
 
 ```bash
-alias -g -- -h='-h | bat --language=help --style=plain'
-alias -g -- --help='--help | bat --language=help --style=plain'
+alias -g -- -h='-h 2>&1 | bat --language=help --style=plain'
+alias -g -- --help='--help 2>&1 | bat --language=help --style=plain'
 ```
 
 This way, you can keep on using `cp --help`, but get colorized help pages.
 
+Be aware that in some cases, `-h` may not be a shorthand of `--help` (for example with `ls`).
+
 Please report any issues with the help syntax in [this repository](https://github.com/victor-gp/cmd-help-sublime-syntax).