Enhance documentation of install command w.r.t. the "Undefined" component.

Inspired-By: Bill Lorensen on the mailing list.

Source/cmInstallCommand.h

@@ -85,7 +85,10 @@ public:
       "with which the install rule is associated, such as \"runtime\" or "
       "\"development\".  During component-specific installation only "
       "install rules associated with the given component name will be "
-      "executed.  During a full installation all components are installed.\n"
+      "executed.  During a full installation all components are installed."
+      " If COMPONENT is not provided a default component \"Unspecified\" is"
+      " created. The default component name may be controlled with the "
+      "CMAKE_INSTALL_DEFAULT_COMPONENT_NAME variable.\n"
       "The RENAME argument specifies a name for an installed file that "
       "may be different from the original file.  Renaming is allowed only "
       "when a single file is installed by the command.\n"