Document that "functions are variables" in SpEL evaluation contexts

Although EvaluationContext defines the API for setting and looking up
variables, the internals of the Spring Expression Language (SpEL)
actually provide explicit support for registering functions as
variables.

This is self-evident in the two registerFunction() variants in
StandardEvaluationContext; however, functions can also be registered as
variables when using the SimpleEvaluationContext.

Since custom functions are also viable in use cases involving the
SimpleEvaluationContext, this commit documents that functions may be
registered in a SimpleEvaluationContext via setVariable().

This commit also explicitly documents the "function as a variable"
behavior in the class-level Javadoc for both StandardEvaluationContext
and SimpleEvaluationContext, as well as in the reference manual.

Closes gh-32258

framework-docs/modules/ROOT/pages/core/expressions/language-ref/functions.adoc

@@ -1,10 +1,26 @@
 [[expressions-ref-functions]]
 = Functions
 
-You can extend SpEL by registering user-defined functions that can be called within the
-expression string. The function is registered through the `EvaluationContext`. The
-following example shows how to register a user-defined function to be invoked via reflection
-(i.e. a `Method`):
+You can extend SpEL by registering user-defined functions that can be called within
+expressions by using the `#functionName(...)` syntax. Functions can be registered as
+variables in `EvaluationContext` implementations via the `setVariable()` method.
+
+[TIP]
+====
+`StandardEvaluationContext` also defines `registerFunction(...)` methods that provide a
+convenient way to register a function as a `java.lang.reflect.Method` or a
+`java.lang.invoke.MethodHandle`.
+====
+
+[WARNING]
+====
+Since functions share a common namespace with
+xref:core/expressions/language-ref/variables.adoc[variables] in the evaluation context,
+care must be taken to ensure that function names and variable names do not overlap.
+====
+
+The following example shows how to register a user-defined function to be invoked via
+reflection using a `java.lang.reflect.Method`:
 
 [tabs]
 ======

framework-docs/modules/ROOT/pages/core/expressions/language-ref/variables.adoc

@@ -20,6 +20,13 @@ characters.
 * dollar sign: `$`
 ====
 
+[WARNING]
+====
+Since variables share a common namespace with
+xref:core/expressions/language-ref/functions.adoc[functions] in the evaluation context,
+care must be taken to ensure that variable names and functions names do not overlap.
+====
+
 The following example shows how to use variables.
 
 [tabs]