chore: more detailed teleport target warning

close #1629

packages/runtime-core/src/components/Teleport.ts

@@ -41,7 +41,10 @@ const resolveTarget = <T = RendererElement>(
       if (!target) {
         __DEV__ &&
           warn(
-            `Failed to locate Teleport target with selector "${targetSelector}".`
+            `Failed to locate Teleport target with selector "${targetSelector}". ` +
+              `Note the target element must exist before the component is mounted - ` +
+              `i.e. the target cannot be rendered by the component itself, and ` +
+              `ideally should be outside of the entire Vue component tree.`
           )
       }
       return target as any