docs/cli: Added command caveats for test and run.

This documentation clearly informs users that system-wide commands in scripts
do not work like they do in npm.

en/docs/cli/run.md

@@ -33,3 +33,16 @@ For example, the following would provide a script named `test` in your package t
   "test": "jest"
 },
 ```
+
+If you have used a package manager like npm previously, you might be used to
+writing system commands directly:
+
+```json
+"scripts": {
+  "test": "echo \"I promise I'll write tests later.\""
+}
+```
+
+Running `yarn run test` will result in an error since `echo` is not guaranteed
+to be a valid command in all environments. This rule also applies to globally
+installed npm packages.

en/docs/cli/test.md

@@ -47,3 +47,30 @@ Hello, world!
 ##### `yarn run test`
 
 `yarn test` is also a shortcut for `yarn run test`.
+
+### Caveats
+
+If you have used a package manager like npm previously, you might be used to
+writing system commands directly:
+
+```json
+"scripts": {
+  "test": "echo \"I promise I'll write tests later.\""
+}
+```
+
+Running `yarn test` will result in an error since `echo` is not guaranteed to be
+a valid command in all environments. This rule also applies to globally
+installed npm packages.
+
+However, locally installed node modules that export an executable command will
+work.
+
+For example, if `jest` is installed using `yarn add`, then the following is
+valid:
+
+```json
+"scripts": {
+  "test": "jest"
+}
+```