MSVC: Document behavior when MSVC_RUNTIME_LIBRARY is not set

By default `CMAKE_MSVC_RUNTIME_LIBRARY` is not set.  Document the
default runtime library selection in that case.  We already test it.

Help/policy/CMP0091.rst

@@ -20,7 +20,9 @@ CMake 3.15 and above prefer to leave the MSVC runtime library selection flags
 out of the default :variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` values and instead
 offer a first-class abstraction.  The :variable:`CMAKE_MSVC_RUNTIME_LIBRARY`
 variable and :prop_tgt:`MSVC_RUNTIME_LIBRARY` target property may be set to
-select the MSVC runtime library.
+select the MSVC runtime library.  If they are not set then CMake uses the
+default value ``MultiThreaded$<$<CONFIG:Debug>:Debug>DLL`` which is
+equivalent to the original flags.
 
 This policy provides compatibility with projects that have not been updated
 to be aware of the abstraction.  The policy setting takes effect as of the

Help/prop_tgt/MSVC_RUNTIME_LIBRARY.rst

@@ -19,6 +19,9 @@ support per-configuration specification.  For example, the code:
 selects for the target ``foo`` a multi-threaded statically-linked runtime
 library with or without debug information depending on the configuration.
 
+If this property is not set then CMake uses the default value
+``MultiThreaded$<$<CONFIG:Debug>:Debug>DLL`` to select a MSVC runtime library.
+
 .. note::
 
   This property has effect only when policy :policy:`CMP0091` is set to ``NEW``

Help/variable/CMAKE_MSVC_RUNTIME_LIBRARY.rst

@@ -20,6 +20,11 @@ support per-configuration specification.  For example, the code:
 selects for all following targets a multi-threaded statically-linked runtime
 library with or without debug information depending on the configuration.
 
+If this variable is not set then the :prop_tgt:`MSVC_RUNTIME_LIBRARY` target
+property will not be set automatically.  If that property is not set then
+CMake uses the default value ``MultiThreaded$<$<CONFIG:Debug>:Debug>DLL``
+to select a MSVC runtime library.
+
 .. note::
 
   This variable has effect only when policy :policy:`CMP0091` is set to ``NEW``