Add small documentation snippet

bin/install-package-tests

@@ -1,7 +1,7 @@
 #!/bin/sh
 
 # Database credentials can be provided via environment variables:
-# - WP_CLI_TEST_DBHOST is the host to use and can include a port, i.e "127.0.0.1:33060" (defaults to "localhost")
+# - WP_CLI_TEST_DBHOST is the host to use and can include a port or a socket after a colon, i.e "127.0.0.1:33060" (defaults to "localhost")
 # - WP_CLI_TEST_DBROOTUSER is the user that has permission to administer databases and users (defaults to "root").
 # - WP_CLI_TEST_DBROOTPASS is the password to use for the above user (defaults to an empty password).
 # - WP_CLI_TEST_DBNAME is the database that the tests run under (defaults to "wp_cli_test").