fix: improve http.client double-wrap message (#20705)

Add a link to the docs.

Author: isaacs

packages/core/src/integrations/http/double-wrap-warning.ts

@@ -7,7 +7,7 @@ const isOtelWrapped = (fn: Function & { __unwrap?: Function }): fn is Function &
 
 // exported for tess
 export const warning =
-  'Double-wrapped http.client detected. Either disable spans in Sentry.httpIntegration, or disable the OpenTelemetry HTTP instrumentation.';
+  'Double-wrapped http.client detected. Either disable spans in Sentry.httpIntegration, or disable the OpenTelemetry HTTP instrumentation. See: https://docs.sentry.io/platforms/javascript/guides/express/opentelemetry/custom-setup/#custom-http-instrumentation';
 
 let didDoubleWrapWarning = false;
 // no-op in non-debug builds