help: mention that double-slashes are not supported in .I

Fixes https://its.cern.ch/jira/browse/ROOT-6431

Author: ferdymercury

lib/MetaProcessor/MetaSema.cpp

@@ -338,7 +338,9 @@ namespace cling {
       "   " << metaString << "U <filename>\t\t- Unloads the given file\n"
       "\n"
       "   " << metaString << "(I|include) [path]\t\t- Shows all include paths. If a path is given,"
-                             "\n\t\t\t\t  adds the path to the include paths.\n"
+                             "\n\t\t\t\t  adds the path to the include paths. Paths with double\n"
+                             "\n\t\t\t\t  slashes // are not supported since they are treated as\n"
+                             "\n\t\t\t\t  C++ comments; use AddIncludePath instead in that case.\n"
       "\n"
       "   " << metaString << "O <level>\t\t\t- Sets the optimization level (0-3)"
                              "\n\t\t\t\t  If no level is given, prints the current setting.\n"