Better documentation of issues.

docs/docs/usage.rst

@@ -25,7 +25,6 @@ Typically if you want to work with one specific Python version, use:
 
 .. autofunction:: parso.load_grammar
 
-
 Grammar methods
 ---------------
 
@@ -37,6 +36,17 @@ issues in it:
     :undoc-members:
 
 
+Error Retrieval
+---------------
+
+|parso| is able to find multiple errors in your source code. Iterating through
+those errors yields the following instances:
+
+.. autoclass:: parso.normalizer.Issue
+    :members:
+    :undoc-members:
+
+
 Utility
 -------