bpo-33571: Improve the glossary description for '...' prompt (GH-6971)

Mention that it can be triggered by triple quotes and after specifying decorators.

Doc/glossary.rst

@@ -14,8 +14,9 @@ Glossary
 
    ``...``
       The default Python prompt of the interactive shell when entering code for
-      an indented code block or within a pair of matching left and right
-      delimiters (parentheses, square brackets or curly braces).
+      an indented code block, when within a pair of matching left and right
+      delimiters (parentheses, square brackets, curly braces or triple quotes),
+      or after specifying a decorator.
 
    2to3
       A tool that tries to convert Python 2.x code to Python 3.x code by