Resolve #3479593 "Migrate alter plugin"
Adds documentation for the following hooks in migrate.api.php:
hook_migrate_destination_info_alter() hook_migrate_source_info_alter() hook_migrate_process_info_alter() hook_migrate_id_map_info_alter() hook_migrate_field_info_alter()
Updates documentation for MigratePluginManager constructur, parameter $type.
Closes #3479593