Add a warning message on the engine selection page if there are less hooks detected that are not supported by the currently selected engine.

Specifically, the following are only supported by leafo/lessphp:


function hook_less_functions() {}

function hook_less_functions_alter() {}

function hook_less_functions_SYSTEM_NAME_alter(&$less_functions) {}

Update documentation pages and less.api.php file to indicate that the hooks are only supported by the legacy leafo/lessphp library.