Define operations before discussing them.

In retrospect, this section ended up being rather confusing
since it assumed the reader understood the idea of an operation
as opposed to a link.  State that concept clearly.

Author: handrews

jsonschema-hyperschema.xml

@@ -468,6 +468,10 @@
                         hint may be added in a future draft (see issue #73 in the GitHub repository).
                     </cref>
                 </t>
+                <t>
+                    An operation is a specific use of a link described by an LDO, after any
+                    template variables have been resolved.
+                </t>
                 <t>
                     There are several ways that a client can use data with a link:
                     <list>