Issue #27076: Doc, comment and test function name spelling fixes

Most fixes to Doc/ and Lib/ directories by Ville Skyttä.
diff --git a/Lib/lib2to3/btm_utils.py b/Lib/lib2to3/btm_utils.py
index 2276dc9..501f834 100644
--- a/Lib/lib2to3/btm_utils.py
+++ b/Lib/lib2to3/btm_utils.py
@@ -216,7 +216,7 @@
                 #reduce to None
                 new_node = None
             elif repeater_node.children[0].value == '+':
-                #reduce to a single occurence i.e. do nothing
+                #reduce to a single occurrence i.e. do nothing
                 pass
             else:
                 #TODO: handle {min, max} repeaters