[pkg/ottl]Godoc comments (open-telemetry#30564)

**Description:** godoc comments added to non-commented public interfaces
in [pkg/ottl] packages

**Link to tracking Issue:** open-telemetry#28893

**Testing:** <Describe what testing was performed and which tests were
added.>

**Documentation:** <Describe the documentation added.>

---------

Co-authored-by: Evan Bradley <11745660+evan-bradley@users.noreply.github.com>
Co-authored-by: Curtis Robert <crobert@splunk.com>
Co-authored-by: Tyler Helmuth <12352919+TylerHelmuth@users.noreply.github.com>

pkg/ottl/expression.go

@@ -27,14 +27,20 @@ func (e Expr[K]) Eval(ctx context.Context, tCtx K) (any, error) {
 	return e.exprFunc(ctx, tCtx)
 }
 
+// Getter resolves a value at runtime without performing any type checking on the value that is returned.
 type Getter[K any] interface {
+	// Get retrieves a value of type 'Any' and returns an error if there are any issues during retrieval.
 	Get(ctx context.Context, tCtx K) (any, error)
 }
 
+// Setter allows setting an untyped value on a predefined field within some data at runtime.
 type Setter[K any] interface {
+	// Set sets a value of type 'Any' and returns an error if there are any issues during the setting process.
 	Set(ctx context.Context, tCtx K, val any) error
 }
 
+// GetSetter is an interface that combines the Getter and Setter interfaces.
+// It should be used to represent the ability to both get and set a value.
 type GetSetter[K any] interface {
 	Getter[K]
 	Setter[K]
@@ -284,6 +290,7 @@ func (g StandardBoolGetter[K]) Get(ctx context.Context, tCtx K) (bool, error) {
 
 // FunctionGetter uses a function factory to return an instantiated function as an Expr.
 type FunctionGetter[K any] interface {
+	// Get returns a function as an Expr[K] built with the provided Arguments
 	Get(args Arguments) (Expr[K], error)
 }