added 'before send transaction' to the common options

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

@@ -226,6 +226,14 @@ The callback typically gets a second argument (called a "hint") which contains t
 
 </ConfigKey>
 
+<ConfigKey name="before-send-transaction">
+
+This function is called with an SDK-specific transaction object, and can return a modified transaction object, or `null` to skip reporting the transaction. This can be used, for instance, for manual PII stripping before sending.
+
+By the time <PlatformIdentifier name="before-send-transaction" /> is executed, all scope data has already been applied to the event. Further modification of the scope won't have any effect.
+
+</ConfigKey>
+
 ## Transport Options
 
 Transports are used to send events to Sentry. Transports can be customized to some degree to better support highly specific deployments.