Add note that autoSessionTracking defaults to true

Reference: https://docs.sentry.io/platforms/javascript/configuration/releases/#sessions

Author: kevinebaugh

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

@@ -138,7 +138,7 @@ Grouping in Sentry is different for events with stack traces and without. As a r
 
 <ConfigKey name="auto-session-tracking" notSupported={["javascript.electron"]}>
 
-When set to `true`, the SDK will send session events to Sentry. This is supported in all browser SDKs, emitting one session per pageload and page navigation to Sentry. In mobile SDKs, when the app goes to the background for longer than 30 seconds, sessions are ended.
+Defaults to `true`. When set to `true`, the SDK will send session events to Sentry. This is supported in all browser SDKs, emitting one session per pageload and page navigation to Sentry. In mobile SDKs, when the app goes to the background for longer than 30 seconds, sessions are ended.
 
 </ConfigKey>