Update README to reflect heartbeatInterval which applies only to async (#2284)

From my reading of code, the sync execute path uses `syncTimeout` and applies it [instantaneously through](https://github.com/StackExchange/StackExchange.Redis/blob/main/src/StackExchange.Redis/ConnectionMultiplexer.cs#L1879) `Monitor.Wait`. 

Looks like the heartbeatInterval only applies to async commands?
https://github.com/StackExchange/StackExchange.Redis/blob/main/src/StackExchange.Redis/PhysicalConnection.cs#L690

Author: rkarthick

docs/Configuration.md

@@ -112,7 +112,7 @@ Additional code-only options:
     - `SocketManager.Shared`: Use a shared dedicated thread pool for _all_ multiplexers (defaults to 10 threads) - best balance for most scenarios.
     - `SocketManager.ThreadPool`: Use the build-in .NET thread pool for scheduling. This can perform better for very small numbers of cores or with large apps on large machines that need to use more than 10 threads (total, across all multiplexers) under load. **Important**: this option isn't the default because it's subject to thread pool growth/starvation and if for example synchronous calls are waiting on a redis command to come back to unblock other threads, stalls/hangs can result. Use with caution, especially if you have sync-over-async work in play.
 - HeartbeatInterval - Default: `1000ms`
-  - Allows running the heartbeat more often which importantly includes timeout evaluation. For example if you have a 50ms command timeout, we're only actually checking it during the heartbeat (once per second by default), so it's possible 50-1050ms pass _before we notice it timed out_. If you want more fidelity in that check and to observe that a server failed faster, you can lower this to run the heartbeat more often to achieve that. 
+  - Allows running the heartbeat more often which importantly includes timeout evaluation for async commands. For example if you have a 50ms async command timeout, we're only actually checking it during the heartbeat (once per second by default), so it's possible 50-1050ms pass _before we notice it timed out_. If you want more fidelity in that check and to observe that a server failed faster, you can lower this to run the heartbeat more often to achieve that. 
   - **Note: heartbeats are not free and that's why the default is 1 second. There is additional overhead to running this more often simply because it does some work each time it fires.**
 
 Tokens in the configuration string are comma-separated; any without an `=` sign are assumed to be redis server endpoints. Endpoints without an explicit port will use 6379 if ssl is not enabled, and 6380 if ssl is enabled.