[docs] Describe using `git grep` in FAQ.
[docs] Describe using `git grep` in FAQ.

Also changed the grep example to use long flag names for clarity.

gregomni/swift,atrick/swift,apple/swift,benlangmuir/swift,gregomni/swift,apple/swift,atrick/swift,rudkx/swift,nathawes/swift,glessard/swift,parkera/swift,nathawes/swift,gregomni/swift,apple/swift,atrick/swift,parkera/swift,nathawes/swift,ahoppen/swift,ahoppen/swift,glessard/swift,nathawes/swift,roambotics/swift,JGiola/swift,xwu/swift,jmgc/swift,ahoppen/swift,hooman/swift,jmgc/swift,rudkx/swift,apple/swift,tkremenek/swift,atrick/swift,xwu/swift,parkera/swift,roambotics/swift,gregomni/swift,tkremenek/swift,nathawes/swift,glessard/swift,tkremenek/swift,benlangmuir/swift,tkremenek/swift,hooman/swift,rudkx/swift,gregomni/swift,benlangmuir/swift,JGiola/swift,apple/swift,JGiola/swift,ahoppen/swift,JGiola/swift,tkremenek/swift,jmgc/swift,hooman/swift,rudkx/swift,xwu/swift,parkera/swift,xwu/swift,atrick/swift,roambotics/swift,rudkx/swift,roambotics/swift,jmgc/swift,ahoppen/swift,ahoppen/swift,hooman/swift,tkremenek/swift,roambotics/swift,JGiola/swift,nathawes/swift,tkremenek/swift,gregomni/swift,hooman/swift,parkera/swift,hooman/swift,benlangmuir/swift,nathawes/swift,benlangmuir/swift,xwu/swift,parkera/swift,parkera/swift,glessard/swift,parkera/swift,jmgc/swift,glessard/swift,jmgc/swift,roambotics/swift,atrick/swift,xwu/swift,jmgc/swift,apple/swift,glessard/swift,rudkx/swift,JGiola/swift,xwu/swift,benlangmuir/swift,hooman/swift