bpo-41224: Document is_annotated() in symtable module and update doc strings (GH-21369)

* Document is_annotate() and update doc strings

* Move quotes to the next line.

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>
diff --git a/Doc/library/symtable.rst b/Doc/library/symtable.rst
index 3efdecb..c9521d6 100644
--- a/Doc/library/symtable.rst
+++ b/Doc/library/symtable.rst
@@ -156,6 +156,10 @@
 
       Return ``True`` if the symbol is local to its block.
 
+    .. method:: is_annotated()
+
+      Return ``True`` if the symbol is annotated.
+
    .. method:: is_free()
 
       Return ``True`` if the symbol is referenced in its block, but not assigned