[component] clarifying lifecycle expectation (#6535)

Extended the documentation of the component to include safe Shutdown expectations.

Co-authored-by: Alex Boten <alex@boten.ca>

component/component.go

@@ -57,6 +57,10 @@ type Component interface {
 	// Shutdown is invoked during service shutdown. After Shutdown() is called, if the component
 	// accepted data in any way, it should not accept it anymore.
 	//
+	// This method must be safe to call:
+	//   - without Start() having been called
+	//   - if the component is in a shutdown state already
+	//
 	// If there are any background operations running by the component they must be aborted before
 	// this function returns. Remember that if you started any long-running background operations from
 	// the Start() method, those operations must be also cancelled. If there are any buffers in the