Help: Remove mention of CMAKE_INSTALL_DO_STRIP

It isn't yet clear whether we want to make this part of the
documented API for the install script. Remove it from the
docs before it appears in an official release for now.

Relates: #18997

Help/manual/cmake.1.rst

@@ -343,7 +343,7 @@ The options are:
   Override the installation prefix, :variable:`CMAKE_INSTALL_PREFIX`.
 
 ``--strip``
-  Strip before installing by setting ``CMAKE_INSTALL_DO_STRIP``.
+  Strip before installing.
 
 ``-v, --verbose``
   Enable verbose output.