Add some doc comments (#23679)

Note that Pager[T] and Poller[T] methods are not goroutine safe.

sdk/azcore/runtime/pager.go

@@ -32,6 +32,7 @@ type PagingHandler[T any] struct {
 }
 
 // Pager provides operations for iterating over paged responses.
+// Methods on this type are not safe for concurrent use.
 type Pager[T any] struct {
 	current   *T
 	handler   PagingHandler[T]

sdk/azcore/runtime/poller.go

@@ -200,6 +200,7 @@ type PollingHandler[T any] interface {
 }
 
 // Poller encapsulates a long-running operation, providing polling facilities until the operation reaches a terminal state.
+// Methods on this type are not safe for concurrent use.
 type Poller[T any] struct {
 	op     PollingHandler[T]
 	resp   *http.Response