doc: distributions: update caveats
doc: distributions: update caveats

rkt on Fedora is still limited because of SELinux and FirewallD but the
situation improved recently. Update the caveats.

sstabellini/rkt,sgotti/rkt,rkt/rkt,kinvolk/rkt,shobhit85/rkt,squeed/rkt,sstabellini/rkt,intelsdi-x/rkt,euank/rkt,lucasallan/rkt,mike1729/rkt,rkt/rkt,ybubnov/rkt,joshix/rkt,kinvolk/rkt,monstermunchkin/rkt,euank/rkt,jonboulle/rkt,jjlakis/rkt,lucasallan/rkt,lucasallan/rkt,monstermunchkin/rkt,tmrts/rkt,rkt/rkt,sgotti/rkt,joshix/rkt,achanda/rkt,intelsdi-x/rkt,markllama/rkt,yifan-gu/rkt,alban/rkt,dgonyeo/rkt,yifan-gu/rocket,Wilybald/rkt,nhlfr/rkt,nhlfr/rkt,kinvolk/rkt,jakon89/rkt,alban/rkt,matthaias/rkt,joshix/rkt,Wilybald/rkt,squeed/rkt,yifan-gu/rkt,shobhit85/rkt,monstermunchkin/rkt,lucasallan/rkt,jakon89/rkt,ybubnov/rkt,jjlakis/rkt,euank/rkt,Shashwatsh/rkt,cab105/rkt,mike1729/rkt,dgonyeo/rkt,ybubnov/rkt,yifan-gu/rocket,yifan-gu/rkt,markllama/rkt,akshaykarle/rkt,sstabellini/rkt,rkt/rkt,shobhit85/rkt,Shashwatsh/rkt,nhlfr/rkt,Wilybald/rkt,kinvolk/rkt,sgotti/rkt,markllama/rkt,cab105/rkt,achanda/rkt,jonboulle/rkt,jonboulle/rkt,squeed/rkt,tmrts/rkt,cab105/rkt,matthaias/rkt,s-urbaniak/rkt,akshaykarle/rkt,coreos/rkt,matthaias/rkt,Shashwatsh/rkt,akshaykarle/rkt,jakon89/rkt,intelsdi-x/rkt,s-urbaniak/rkt,Wilybald/rkt,joshix/rkt,coreos/rkt,mike1729/rkt,coreos/rkt,jjlakis/rkt,tmrts/rkt,s-urbaniak/rkt,achanda/rkt,yifan-gu/rocket,alban/rkt,dgonyeo/rkt