Update javascript.mdx (#1769)

Updating to be explicit about how start_session_recording() behaves

Co-authored-by: Patrick Mackle <patrickthemackle@gmail.com>

Author: djmixpanel

pages/docs/tracking-methods/sdks/javascript.mdx

@@ -958,7 +958,7 @@ We give our customers full control to customize when and where they capture sess
 ```javascript
 mixpanel.start_session_recording()
 ```
-This will have no effect if replay data collection is in progress. 
+This will force recording to begin, overriding any value defined in `record_sessions_percent`. This will have no effect if replay data collection is in progress. 
 
 This is optional, and can be used primarily to programmatically start and stop recording, or exclude something specific from recording. We recommend using the [sampling method](/docs/tracking-methods/sdks/javascript#sampling-method) detailed above unless you need to customize when you capture replay data.