Documented backpressure handling option for .NET

Resolves getsentry/sentry-dotnet#4452
- getsentry/sentry-dotnet#4452

Author: jamescrosswell

docs/platforms/dotnet/common/configuration/options.mdx

@@ -69,6 +69,16 @@ See the <PlatformLink to="/enriching-events/scopes/">Scopes and Hubs documentati
 
 </ConfigKey>
 
+<ConfigKey name="enable-backpressure-handling">
+
+Specifies whether backpressure handling should be enabled or not.
+
+When enabled, the sample rate for traces and errors will be reduced automatically when the SDK detects issues sending events to Sentry. Once the system is healthy again, the sample rate will be restored to the originally configured value.
+
+Currently defaults to `false` (i.e. disabled), but will be enabled by default in a future version.
+
+</ConfigKey>
+
 <ConfigKey name="debug">
 
 Turns debug mode on or off. If debug is enabled SDK will attempt to print out useful debugging information if something goes wrong with sending the event. The default is always `false`. It's generally not recommended to turn it on in production, though turning `debug` mode on will not cause any safety concerns.