Add a help entry and documentation for @ character (bug #52771). expr.txi: Add sub-node "Access via Handle" to the "Calling Functions" node. Don't indent a paragraph that refers to the previous example. Provide the body for the "Access via Handle" node and include an '@' opindex entry qualified by "function handle". Mention that '@' is also used for class methods and reference the "Object Oriented Programming" chapter. oop.txi: Add an opindex reference to '@' qualified by "class methods" so a quick index link is created. op-kw-docs: Create an operator index entry for '@'. Mention that '@' is also used for class methods and suggest seeing the "Object Oriented Programming" chapter of the manual. octave.texi: Add sub-node "Access via Handle" to sub-menu list.

--- a/doc/interpreter/oop.txi	Wed Jan 03 22:30:54 2018 -0800
+++ b/doc/interpreter/oop.txi	Tue Jan 02 15:29:10 2018 -0600
@@ -81,6 +81,7 @@
 a = [a0, a1, a2, @dots{}, an];
 @end example
 
+@opindex @@ class methods
 This is a sufficient specification to begin writing the constructor for the
 polynomial class.  All object oriented classes in Octave must be located in a
 directory that is the name of the class prepended with the @samp{@@} symbol.