[DOCS] Add rollup API prereqs

docs/reference/rollup/apis/rollup-api.asciidoc

@@ -51,9 +51,14 @@ POST /my-index-000001/_rollup/rollup-my-index-000001
 [[rollup-api-prereqs]]
 ==== {api-prereq-title}
 
-If the {es} {security-features} are enabled, you must have the `manage` or
+* If the {es} {security-features} are enabled, you must have the `manage` or
 `manage_rollup` cluster privilege to use this API. See <<security-privileges>>.
 
+* You can only roll up an index that contains:
+
+** A <<date,`date`>> or <<date_nanos,`date_nanos`>> timestamp field
+** One or more <<number,numeric>> fields
+
 [[rollup-api-path-params]]
 ==== {api-path-parms-title}