doc: slightly more convenient plugin loading.

Less typing and less name duplication, but hides the actual load error
from the user. The error is printed on stdout anyway and the user
doesn't care anyway in 90% cases.

doc/plugins.dox

@@ -129,12 +129,12 @@ lifetime of all plugin instances created from it.
 // ...
 
 {
-    PluginManager::Manager<Trade::AbstractImporter> manager(MAGNUM_PLUGINS_IMPORTER_DIR);
+    PluginManager::Manager<Trade::AbstractImporter> manager{MAGNUM_PLUGINS_IMPORTER_DIR};
-    if(!(manager.load("TgaImporter") & PluginManager::LoadState::Loaded)) {
+    std::unique_ptr<Trade::AbstractImporter> tgaImporter = manager.loadAndInstantiate("TgaImporter");
-        Error() << "Cannot load TgaImporter plugin from" << manager.pluginDirectory();
+    if(!tgaImporter) {
+        Error() << "Cannot load importer plugin from" << manager.pluginDirectory();
         std::exit(1);
     }
-    std::unique_ptr<Trade::AbstractImporter> tgaImporter = manager.instance("TgaImporter");
 
     // Use the plugin...