Help: mention CMAKE_SKIP_RPATH in the RPATH docs

CMAKE_SKIP_RPATH is now mentioned in the docs for the BUILD_RPATH
and INSTALL_RPATH target properties.

Help/prop_tgt/BUILD_RPATH.rst

@@ -23,6 +23,9 @@ This property supports
 
 Other settings that affect the build-tree runtime path include:
 
+* The :variable:`CMAKE_SKIP_RPATH` variable completely disables runtime
+  paths in both the build tree and install tree.
+
 * The :prop_tgt:`SKIP_BUILD_RPATH` target property disables setting any
   runtime path in the build tree.
 

Help/prop_tgt/INSTALL_RPATH.rst

@@ -22,6 +22,8 @@ the :prop_tgt:`BUILD_WITH_INSTALL_RPATH` target property.
 
 For handling toolchain-dependent RPATH entries the
 :prop_tgt:`INSTALL_REMOVE_ENVIRONMENT_RPATH` can be used.
+Runtime paths can be disabled completely via the :variable:`CMAKE_SKIP_RPATH`
+variable.
 
 Because the rpath may contain ``${ORIGIN}``, which coincides with CMake syntax,
 the contents of ``INSTALL_RPATH`` are properly escaped in the