Address PR comments

Author: Theofilos Manitaras

docs/migration_2_to_3.rst

@@ -23,7 +23,7 @@ However, you can convert any old configuration file using the command line optio
 
 .. warning::
    .. versionchanged:: 3.4
-      The old configuration syntax in no longer supported.
+      The old configuration syntax in no longer supported and it will not be automatically converted by the `-C` option.
 
 
 Another important change is that default locations for looking up a configuration file has changed (see `Configuring ReFrame for Your Site <configure.html>`__ for more details).
@@ -35,7 +35,7 @@ You have to move it to any of the default settings locations or set the correspo
 
 
 Configuration conversion limitations
-================================
+====================================
 
 ReFrame does a pretty good job in converting correctly your old configuration files, but there are some limitations:
 

reframe/core/config.py

@@ -226,8 +226,9 @@ def _create_from_python(cls, filename):
         if hasattr(mod, 'settings'):
             # Looks like an old style config
             raise ConfigError(
-                f"the syntax of the configuration file '{filename}' "
-                f"is no longer supported"
+                f"the syntax of the configuration file {filename!r} "
+                f"is no longer supported; please convert it using the "
+                f"'--upgrade-config-file' option"
             )
 
         mod = util.import_module_from_file(filename)