Fix type hint of fields attribute.
Fix type hint of fields attribute.

Summary: Also, add a comment about it.

Reviewed By: sbalabanov

fbshipit-source-id: de5ce7c13d

JoelMarcey/buck,SeleniumHQ/buck,Addepar/buck,brettwooldridge/buck,romanoid/buck,JoelMarcey/buck,JoelMarcey/buck,zpao/buck,shs96c/buck,kageiit/buck,Addepar/buck,shs96c/buck,brettwooldridge/buck,shs96c/buck,kageiit/buck,JoelMarcey/buck,SeleniumHQ/buck,facebook/buck,SeleniumHQ/buck,romanoid/buck,Addepar/buck,brettwooldridge/buck,brettwooldridge/buck,romanoid/buck,shs96c/buck,SeleniumHQ/buck,facebook/buck,SeleniumHQ/buck,rmaz/buck,rmaz/buck,kageiit/buck,nguyentruongtho/buck,shs96c/buck,zpao/buck,nguyentruongtho/buck,facebook/buck,Addepar/buck,Addepar/buck,JoelMarcey/buck,brettwooldridge/buck,zpao/buck,brettwooldridge/buck,romanoid/buck,SeleniumHQ/buck,zpao/buck,nguyentruongtho/buck,JoelMarcey/buck,SeleniumHQ/buck,kageiit/buck,SeleniumHQ/buck,shs96c/buck,Addepar/buck,brettwooldridge/buck,facebook/buck,Addepar/buck,JoelMarcey/buck,brettwooldridge/buck,brettwooldridge/buck,nguyentruongtho/buck,facebook/buck,JoelMarcey/buck,shs96c/buck,kageiit/buck,SeleniumHQ/buck,rmaz/buck,rmaz/buck,JoelMarcey/buck,JoelMarcey/buck,shs96c/buck,nguyentruongtho/buck,facebook/buck,Addepar/buck,romanoid/buck,rmaz/buck,romanoid/buck,brettwooldridge/buck,shs96c/buck,zpao/buck,rmaz/buck,JoelMarcey/buck,shs96c/buck,shs96c/buck,nguyentruongtho/buck,Addepar/buck,SeleniumHQ/buck,romanoid/buck,shs96c/buck,rmaz/buck,romanoid/buck,nguyentruongtho/buck,rmaz/buck,SeleniumHQ/buck,romanoid/buck,brettwooldridge/buck,Addepar/buck,rmaz/buck,brettwooldridge/buck,rmaz/buck,romanoid/buck,rmaz/buck,romanoid/buck,romanoid/buck,zpao/buck,romanoid/buck,JoelMarcey/buck,Addepar/buck,rmaz/buck,Addepar/buck,JoelMarcey/buck,kageiit/buck,shs96c/buck,Addepar/buck,SeleniumHQ/buck,SeleniumHQ/buck,zpao/buck,facebook/buck,rmaz/buck,kageiit/buck,brettwooldridge/buck