feat: make oninit public (#307)

* feat: make oninit public

* Update docs

Author: jooohhn

src/amplitude-client.js

@@ -491,8 +491,8 @@ AmplitudeClient.prototype.isNewSession = function isNewSession() {
 };
 
 /**
- * Store callbacks to call after init
- * @private
+ * Add callbacks to call after init. Useful for users who load Amplitude through a snippet.
+ * @public
  */
 AmplitudeClient.prototype.onInit = function (callback) {
   if (this._isInitialized) {