Update cache ref docs re 'args' vs 'params' naming

Prior to this change, the caching reference docs referred to
'root.params', whereas the actual naming should be 'root.args'. This
naming was also reflected in the "#p" syntax for specifying method args.

This change updates the documentation to refer to 'root.args' properly
and also adds "#a" syntax for specifying method arguments more
intuitively. Note that "#p" syntax remains in place as an alias for
backward compatibility.

Issue: SPR-8938

org.springframework.context/src/main/java/org/springframework/cache/interceptor/LazyParamAwareEvaluationContext.java

@@ -97,6 +97,7 @@ class LazyParamAwareEvaluationContext extends StandardEvaluationContext {
 
 		// save arguments as indexed variables
 		for (int i = 0; i < this.args.length; i++) {
+			setVariable("a" + i, this.args[i]);
 			setVariable("p" + i, this.args[i]);
 		}