[EventEngine] Add advice against blocking work in callbacks (#32397)

<!--

If you know who should review your pull request, please assign it to
that
person, otherwise the pull request would get assigned randomly.

If your pull request is for a specific language, please add the
appropriate
lang label.

-->

include/grpc/event_engine/event_engine.h

@@ -33,6 +33,11 @@ namespace grpc_event_engine {
 namespace experimental {
 
 ////////////////////////////////////////////////////////////////////////////////
+/// The EventEngine Interface
+///
+/// Overview
+/// --------
+///
 /// The EventEngine encapsulates all platform-specific behaviors related to low
 /// level network I/O, timers, asynchronous execution, and DNS resolution.
 ///
@@ -44,7 +49,8 @@ namespace experimental {
 ///
 /// A default cross-platform EventEngine instance is provided by gRPC.
 ///
-/// LIFESPAN AND OWNERSHIP
+/// Lifespan and Ownership
+/// ----------------------
 ///
 /// gRPC takes shared ownership of EventEngines via std::shared_ptrs to ensure
 /// that the engines remain available until they are no longer needed. Depending
@@ -71,6 +77,19 @@ namespace experimental {
 ///    std::unique_ptr<Server> server(builder.BuildAndStart());
 ///    server->Wait();
 ///
+///
+/// Blocking EventEngine Callbacks
+/// -----------------------------
+///
+/// Doing blocking work in EventEngine callbacks is generally not advisable.
+/// While gRPC's default EventEngine implementations have some capacity to scale
+/// their thread pools to avoid starvation, this is not an instantaneous
+/// process. Further, user-provided EventEngines may not be optimized to handle
+/// excessive blocking work at all.
+///
+/// *Best Practice* : Occasional blocking work may be fine, but we do not
+/// recommend running a mostly blocking workload in EventEngine threads.
+///
 ////////////////////////////////////////////////////////////////////////////////
 class EventEngine : public std::enable_shared_from_this<EventEngine> {
  public: