Update WebView#clearClientCertPreferences docs.

Remove the reference to the specific ACTION_STORAGE_CHANGED intent as
other intents are used instead on newer OS versions; just note that it's
cleared automatically when the keychain is updated and don't specify the
exact mechanism.

Change-Id: Ic677832a1384e0eb2498d06e7aa34507fd2e7278
Fixes: 30371615
Test: make offline-sdk-docs