Update fibers GDB docs
Update fibers GDB docs

Reviewed By: yfeldblum

Differential Revision: D4323918

fbshipit-source-id: 8dcd4ef3629fff061a9f149093ef1fe087f1c7e2

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