doc/modules/http.hsts: Document :store return value

Author: daurnimator

doc/modules/http.hsts.md

@@ -16,6 +16,8 @@ Creates and returns a copy of a store.
 
 Add new directives to the store about the given `host`. `directives` should be a table of directives, which *must* include the key `"max-age"`.
 
+Returns a boolean indicating if the item was accepted.
+
 
 ### `hsts_store:remove(host)` <!-- --> {#http.hsts:remove}