Provide help String for `xmode`.

Closes  #8866

IPython/core/interactiveshell.py

@@ -389,7 +389,9 b' class InteractiveShell(SingletonConfigurable):'
     separate_out2 = SeparateUnicode('').tag(config=True)
     wildcards_case_sensitive = Bool(True).tag(config=True)
     xmode = CaselessStrEnum(('Context','Plain', 'Verbose'),
-                            default_value='Context').tag(config=True)
+                            default_value='Context',
+                            help="Switch modes for the IPython exception handlers."
+                            ).tag(config=True)
 
     # Subcomponents of InteractiveShell
     alias_manager = Instance('IPython.core.alias.AliasManager', allow_none=True)