Introduce description field for function & parameters

cty/function/argument.go

@@ -10,6 +10,9 @@ type Parameter struct {
 	// value, but callers may use it for documentation, etc.
 	Name string
 
+	// Description is an optional description for the argument.
+	Description string
+
 	// A type that any argument for this parameter must conform to.
 	// cty.DynamicPseudoType can be used, either at top-level or nested
 	// in a parameterized type, to indicate that any type should be

cty/function/function.go

@@ -14,6 +14,9 @@ type Function struct {
 // Spec is the specification of a function, used to instantiate
 // a new Function.
 type Spec struct {
+	// Description is an optional description for the function specification.
+	Description string
+
 	// Params is a description of the positional parameters for the function.
 	// The standard checking logic rejects any calls that do not provide
 	// arguments conforming to this definition, freeing the function
@@ -344,3 +347,8 @@ func (f Function) VarParam() *Parameter {
 	ret := *f.spec.VarParam
 	return &ret
 }
+
+// Description returns a human-readable description of the function.
+func (f Function) Description() string {
+	return f.spec.Description
+}