feat: Add BrowserTracing heartbeatInterval option description

Author: Lms24

src/platforms/javascript/common/performance/instrumentation/automatic-instrumentation.mdx

@@ -76,6 +76,12 @@ The idle time, measured in ms, to wait until the transaction will be finished. T
 
 The default is `1000`.
 
+### heartbeatInterval
+
+The time, measured in ms, one heartbeat takes. If no new spans were started or no open spans finished within **three heartbeats**, the transaction will be finished. The heartbeat count restarts whenever a new span is created or an open span is finished.
+
+The default is `5000`.
+
 ### startTransactionOnLocationChange
 
 This flag enables or disables creation of `navigation` transaction on history changes.
@@ -100,19 +106,19 @@ This option flags transactions when tabs are moved to the background with "cance
 
 The default is `true`.
 
+### \_experiments
 
-### _experiments
-
-This is an object containing experimental flags for features that may involve breaking changes that haven't yet landed in a new major version. 
+This is an object containing experimental flags for features that may involve breaking changes that haven't yet landed in a new major version.
 
 The default is
+
 ```
 {
   enableLongTasks: true
 }
 ```
 
-#### _experiments.enableLongTasks
+#### \_experiments.enableLongTasks
 
 This experimental option determines whether spans for long tasks automatically get created.