Help: Add missing versionadded to *CROSSCOMPILING_EMULATOR

The list capability was added in CMake 3.15 by commit fec441ec17 (Teach
CROSSCOMPILING_EMULATOR to support arguments, 2019-05-30,
v3.15.0-rc1~6^2), but the documentation did not indicate the version
that added it.

Help/prop_tgt/CROSSCOMPILING_EMULATOR.rst

@@ -8,9 +8,10 @@ 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.
 
-If this property contains a :ref:`semicolon-separated list <CMake Language
-Lists>`, then the first value is the command and remaining values are its
-arguments.
+.. versionadded:: 3.15
+  If this property contains a :ref:`semicolon-separated list <CMake Language
+  Lists>`, then the first value is the command and remaining values are its
+  arguments.
 
 This property is initialized by the value of the
 :variable:`CMAKE_CROSSCOMPILING_EMULATOR` variable if it is set when a target

Help/variable/CMAKE_CROSSCOMPILING_EMULATOR.rst

@@ -7,9 +7,10 @@ This variable is only used when :variable:`CMAKE_CROSSCOMPILING` is on. It
 should point to a command on the host system that can run executable built
 for the target system.
 
-If this variable contains a :ref:`semicolon-separated list <CMake Language
-Lists>`, then the first value is the command and remaining values are its
-arguments.
+.. versionadded:: 3.15
+  If this variable contains a :ref:`semicolon-separated list <CMake Language
+  Lists>`, then the first value is the command and remaining values are its
+  arguments.
 
 The command will be used to run :command:`try_run` generated executables,
 which avoids manual population of the ``TryRunResults.cmake`` file.