Help: Make note of ninja -f argument in Ninja Multi-Config docs

Help/generator/Ninja Multi-Config.rst

@@ -11,7 +11,8 @@ Unlike the :generator:`Ninja` generator, ``Ninja Multi-Config`` generates
 multiple configurations at once with :variable:`CMAKE_CONFIGURATION_TYPES`
 instead of only one configuration with :variable:`CMAKE_BUILD_TYPE`. One
 ``build-<Config>.ninja`` file will be generated for each of these
-configurations (with ``<Config>`` being the configuration name.) No
+configurations (with ``<Config>`` being the configuration name.) These files
+are intended to be run with ``ninja -f build-<Config>.ninja``. No
 ``build.ninja`` file is generated by default (see below for how to generate
 it.)