Updating javadocs.
Updating javadocs.

Summary: Updating javadocs for LoginActivity.

Test Plan: builds

Reviewers: mmarucheck, clang, jacl

Reviewed By: mmarucheck

CC: msdkexp@

Differential Revision: https://phabricator.fb.com/D652420

Task ID: 1946899

lizhi5753186/facebook-android-sdk,couchbaselabs/facebook-android-sdk,MKrishtop/facebook-android-sdk,mindjolt/facebook-android-sdk,RabbleApp/facebook-android-sdk,taisukeoe/facebook-android-sdk,CristianOliveiraDaRosa/facebook-android-sdk,scalio/facebook-android-sdk,ribot/facebook-android-sdk,minhduc3404/facebookSdk,bitcellar-labs/facebook-android-sdk,Flutterbee/facebook-android-sdk,MarkHappy/Facebook_SDK,couchbaselabs/facebook-android-sdk,PioneerLab/facebook-android-sdk,Limbika/facebook-android-sdk,lizhi5753186/facebook-android-sdk,quizlet/facebook-android-sdk,TechSmith/facebook-android-sdk,TechSmith/facebook-android-sdk,satoshi-okawa/facebook-android-sdk-test1,bitcellar-labs/facebook-android-sdk,Limbika/facebook-android-sdk,MKrishtop/facebook-android-sdk,taisukeoe/facebook-android-sdk,MarkHappy/Facebook_SDK,TimeIncOSS/facebook-android-sdk,satoshi-okawa/facebook-android-sdk-test1,RabbleApp/facebook-android-sdk,scalio/facebook-android-sdk,mindjolt/facebook-android-sdk,minhduc3404/facebookSdk,TimeIncOSS/facebook-android-sdk,quizlet/facebook-android-sdk,CristianOliveiraDaRosa/facebook-android-sdk,PioneerLab/facebook-android-sdk,ribot/facebook-android-sdk,GrioSF/facebook-android-sdk,Flutterbee/facebook-android-sdk,GrioSF/facebook-android-sdk