rust: update buckconfig docs
rust: update buckconfig docs

Summary: Add rustc_flags to rust section of buckconfig

Test Plan: Read

Reviewed By: bolinfest

fbshipit-source-id: 2d651a3

k21/buck,sdwilsh/buck,facebook/buck,dsyang/buck,justinmuller/buck,darkforestzero/buck,vschs007/buck,dsyang/buck,marcinkwiatkowski/buck,vschs007/buck,k21/buck,clonetwin26/buck,grumpyjames/buck,SeleniumHQ/buck,LegNeato/buck,romanoid/buck,clonetwin26/buck,darkforestzero/buck,Addepar/buck,JoelMarcey/buck,dsyang/buck,romanoid/buck,sdwilsh/buck,marcinkwiatkowski/buck,robbertvanginkel/buck,zhan-xiong/buck,brettwooldridge/buck,SeleniumHQ/buck,justinmuller/buck,Addepar/buck,shs96c/buck,nguyentruongtho/buck,ilya-klyuchnikov/buck,k21/buck,rmaz/buck,zpao/buck,zhan-xiong/buck,robbertvanginkel/buck,romanoid/buck,daedric/buck,marcinkwiatkowski/buck,shybovycha/buck,zhan-xiong/buck,zpao/buck,shs96c/buck,dsyang/buck,davido/buck,kageiit/buck,robbertvanginkel/buck,davido/buck,SeleniumHQ/buck,k21/buck,rmaz/buck,darkforestzero/buck,SeleniumHQ/buck,shybovycha/buck,zhan-xiong/buck,Addepar/buck,kageiit/buck,daedric/buck,JoelMarcey/buck,Addepar/buck,daedric/buck,vschs007/buck,rmaz/buck,dsyang/buck,davido/buck,sdwilsh/buck,robbertvanginkel/buck,marcinkwiatkowski/buck,rmaz/buck,vschs007/buck,ilya-klyuchnikov/buck,shybovycha/buck,LegNeato/buck,clonetwin26/buck,romanoid/buck,brettwooldridge/buck,facebook/buck,LegNeato/buck,JoelMarcey/buck,shybovycha/buck,zpao/buck,daedric/buck,romanoid/buck,brettwooldridge/buck,davido/buck,daedric/buck,darkforestzero/buck,ilya-klyuchnikov/buck,shybovycha/buck,marcinkwiatkowski/buck,SeleniumHQ/buck,zpao/buck,dsyang/buck,SeleniumHQ/buck,clonetwin26/buck,zhan-xiong/buck,davido/buck,LegNeato/buck,LegNeato/buck,ilya-klyuchnikov/buck,sdwilsh/buck,robbertvanginkel/buck,shs96c/buck,marcinkwiatkowski/buck,shybovycha/buck,zhan-xiong/buck,justinmuller/buck,facebook/buck,romanoid/buck,JoelMarcey/buck,grumpyjames/buck,JoelMarcey/buck,marcinkwiatkowski/buck,rmaz/buck,vschs007/buck,kageiit/buck,darkforestzero/buck,nguyentruongtho/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,davido/buck,zpao/buck,ilya-klyuchnikov/buck,JoelMarcey/buck,marcinkwiatkowski/buck,darkforestzero/buck,Addepar/buck,marcinkwiatkowski/buck,Addepar/buck,LegNeato/buck,robbertvanginkel/buck,brettwooldridge/buck,sdwilsh/buck,Addepar/buck,JoelMarcey/buck,justinmuller/buck,Addepar/buck,facebook/buck,zhan-xiong/buck,sdwilsh/buck,k21/buck,davido/buck,Addepar/buck,SeleniumHQ/buck,shybovycha/buck,Addepar/buck,brettwooldridge/buck,nguyentruongtho/buck,justinmuller/buck,davido/buck,clonetwin26/buck,sdwilsh/buck,marcinkwiatkowski/buck,justinmuller/buck,daedric/buck,romanoid/buck,sdwilsh/buck,sdwilsh/buck,ilya-klyuchnikov/buck,davido/buck,rmaz/buck,brettwooldridge/buck,brettwooldridge/buck,justinmuller/buck,romanoid/buck,justinmuller/buck,grumpyjames/buck,clonetwin26/buck,Addepar/buck,grumpyjames/buck,romanoid/buck,shs96c/buck,kageiit/buck,rmaz/buck,robbertvanginkel/buck,SeleniumHQ/buck,nguyentruongtho/buck,k21/buck,daedric/buck,Addepar/buck,vschs007/buck,grumpyjames/buck,dsyang/buck,grumpyjames/buck,rmaz/buck,nguyentruongtho/buck,clonetwin26/buck,JoelMarcey/buck,clonetwin26/buck,shs96c/buck,davido/buck,k21/buck,daedric/buck,vschs007/buck,JoelMarcey/buck,daedric/buck,JoelMarcey/buck,robbertvanginkel/buck,zhan-xiong/buck,justinmuller/buck,darkforestzero/buck,grumpyjames/buck,vschs007/buck,daedric/buck,clonetwin26/buck,clonetwin26/buck,shs96c/buck,kageiit/buck,dsyang/buck,daedric/buck,ilya-klyuchnikov/buck,romanoid/buck,zhan-xiong/buck,k21/buck,daedric/buck,shybovycha/buck,dsyang/buck,shs96c/buck,dsyang/buck,marcinkwiatkowski/buck,zhan-xiong/buck,grumpyjames/buck,romanoid/buck,clonetwin26/buck,rmaz/buck,SeleniumHQ/buck,facebook/buck,LegNeato/buck,rmaz/buck,marcinkwiatkowski/buck,SeleniumHQ/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,brettwooldridge/buck,shs96c/buck,LegNeato/buck,ilya-klyuchnikov/buck,brettwooldridge/buck,clonetwin26/buck,k21/buck,JoelMarcey/buck,shs96c/buck,clonetwin26/buck,LegNeato/buck,nguyentruongtho/buck,JoelMarcey/buck,brettwooldridge/buck,sdwilsh/buck,sdwilsh/buck,dsyang/buck,brettwooldridge/buck,darkforestzero/buck,vschs007/buck,rmaz/buck,vschs007/buck,rmaz/buck,sdwilsh/buck,davido/buck,LegNeato/buck,darkforestzero/buck,k21/buck,brettwooldridge/buck,zpao/buck,zpao/buck,darkforestzero/buck,kageiit/buck,Addepar/buck,dsyang/buck,LegNeato/buck,shs96c/buck,shybovycha/buck,grumpyjames/buck,ilya-klyuchnikov/buck,shybovycha/buck,grumpyjames/buck,LegNeato/buck,rmaz/buck,justinmuller/buck,robbertvanginkel/buck,marcinkwiatkowski/buck,shybovycha/buck,ilya-klyuchnikov/buck,SeleniumHQ/buck,grumpyjames/buck,dsyang/buck,k21/buck,darkforestzero/buck,robbertvanginkel/buck,shybovycha/buck,grumpyjames/buck,zhan-xiong/buck,shs96c/buck,SeleniumHQ/buck,k21/buck,vschs007/buck,JoelMarcey/buck,robbertvanginkel/buck,romanoid/buck,justinmuller/buck,LegNeato/buck,justinmuller/buck,davido/buck,zhan-xiong/buck,darkforestzero/buck,SeleniumHQ/buck,vschs007/buck,daedric/buck,facebook/buck,romanoid/buck,robbertvanginkel/buck,sdwilsh/buck,darkforestzero/buck,nguyentruongtho/buck,shybovycha/buck,davido/buck,kageiit/buck,justinmuller/buck,shs96c/buck,vschs007/buck,k21/buck,facebook/buck,zhan-xiong/buck,shs96c/buck