Help: Cross compile variable used as initial value for target property

Variable CMAKE_CROSSCOMPILING_EMULATOR is used to initialize
CROSSCOMPILING_EMULATOR.

Issue: #16785

Help/prop_tgt/CROSSCOMPILING_EMULATOR.rst

@@ -5,3 +5,7 @@ Use the given emulator to run executables created when crosscompiling.
 This command will be added as a prefix to :command:`add_test`,
 :command:`add_custom_command`, and :command:`add_custom_target` commands
 for built target system executables.
+
+This property is initialized by the value of the
+:variable:`CMAKE_CROSSCOMPILING_EMULATOR` variable if it is set when a target
+is created.