beforeSend clarification (#829)

is this in dev docs somewhere already?

Author: smeubank

develop-docs/sdk/features.mdx

@@ -163,7 +163,7 @@ Whenever possible, avoid adding the attachment altogether if taking the screensh
 
 ## Before-Send Hook
 
-Hook called with the event (and on some platforms the hint) that allow the user to decide whether an event should be sent or not.  This can also be used to further modify the event.
+Hook called with the event (and on some platforms the hint) that allows the user to decide whether an event should be sent or not.  This can also be used to further modify the event. This only works for `error` events. For `transactions` it is recommended to have `beforeSendTransaction` implemented in SDKs.
 
 ## Before-Breadcrumb Hook