Help: Explain new behavior of cmake --build in Ninja Multi-Config

Help/generator/Ninja Multi-Config.rst

@@ -15,6 +15,11 @@ configurations (with ``<Config>`` being the configuration name.) No
 ``build.ninja`` file is generated by default (see below for how to generate
 it.)
 
+``cmake --build . --config <Config>`` will always use ``build-<Config>.ninja``
+to build. If no ``--config`` argument is specified, ``cmake --build .`` will
+default to ``build-Debug.ninja``, unless a ``build.ninja`` is generated (see
+below), in which case that will be used instead.
+
 Each ``build-<Config>.ninja`` file contains ``<target>`` targets as well as
 ``<target>:<Config>`` targets, where ``<Config>`` is the same as the
 configuration specified in ``build-<Config>.ninja`` Additionally, if