Document support for varargs invocations in SpEL

[sbrannen]

As discussed in #33315 (comment) and subsequent comments in that issue, the language reference for the Spring Expression Language (SpEL) currently does not document support for varargs method, constructor, and function invocations.

We should add notes to the corresponding sections (Constructors, Methods, Functions) to officially document our varargs support in SpEL, pointing out that a java.util.List will be converted to an array for the purpose of varargs invocations and that a varargs invocation in SpEL should otherwise behave the same as a varargs invocation in standard Java (where applicable).