instrumentation: Add experimental notes to docs

Help/command/cmake_instrumentation.rst

@@ -3,6 +3,11 @@ cmake_instrumentation
 
 .. versionadded:: 4.0
 
+.. note::
+
+   This command is only available when experimental support for instrumentation
+   has been enabled by the ``CMAKE_EXPERIMENTAL_INSTRUMENTATION`` gate.
+
 Enables interacting with the
 :manual:`CMake Instrumentation API <cmake-instrumentation(7)>`.
 

Help/manual/cmake-instrumentation.7.rst

@@ -12,6 +12,11 @@ cmake-instrumentation(7)
 Introduction
 ============
 
+.. note::
+
+   This feature is only available when experimental support for instrumentation
+   has been enabled by the ``CMAKE_EXPERIMENTAL_INSTRUMENTATION`` gate.
+
 The CMake Instrumentation API allows for the collection of timing data, target
 information and system diagnostic information during the configure, generate,
 build, test and install steps for a CMake project.