Updating documentation for folly::Synchronized to include upgradable
Updating documentation for folly::Synchronized to include upgradable

Summary:
This diff updates the documentation for folly::Synchronized to include
upgradable locking

Reviewed By: yfeldblum

Differential Revision: D3740983

fbshipit-source-id: d201fcfa6f62ce168125d2a9caa096e079446efa

facebook/folly,charsyam/folly,floxard/folly,Orvid/folly,floxard/folly,Orvid/folly,rklabs/folly,floxard/folly,Orvid/folly,facebook/folly,rklabs/folly,rklabs/folly,charsyam/folly,facebook/folly,charsyam/folly,Orvid/folly,facebook/folly,charsyam/folly,facebook/folly,rklabs/folly,Orvid/folly,floxard/folly,rklabs/folly,floxard/folly