ref(js): Add deprecation note to startTransaction documentation (#8945)

Author: Lms24

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

@@ -38,7 +38,15 @@ Sentry maintains a [list of well known span operations](https://develop.sentry.d
 
 <PlatformContent includePath="performance/span-operations" />
 
-## Start Transaction
+## (Deprecated) Start Transaction
+
+<Alert level="warning" title="Deprecation">
+
+The `startTransaction` function is deprecated and will be removed in version 8 of the SDK.
+
+To migrate, use the span starting functions documented above as a replacement.
+
+</Alert>
 
 The root span (the span that is the parent of all other spans) is known as a **transaction** in Sentry. Transactions can be accessed and created separately if you need more control over your timing data or if you use a version of the SDK that doesn't support the top-level span APIs.