Drop text about 'legacy methods' From RestrictedApi methods

PiperOrigin-RevId: 621319804

docs/bugpattern/RestrictedApi.md

@@ -1,2 +1,11 @@
-Calls to APIs marked @RestrictedApi are prohibited without a corresponding
+Calls to APIs marked `@RestrictedApi` are prohibited without a corresponding
 allowlist annotation.
+
+The intended use-case for `@RestrictedApi` is to restrict calls to annotated
+methods so that each usage of those APIs must be reviewed separately. For
+example, an API might lead to security bugs unless the programmer uses it
+correctly.
+
+See the
+[javadoc for `@RestrictedApi`](https://errorprone.info/api/latest/com/google/errorprone/annotations/RestrictedApi.html)
+for more details.