add known issue for swarm admin UI bug (#5399)
add known issue for swarm admin UI bug (#5399)

* add known issue for swarm admin UI bug

* elaborate on new known issue

londoncalling/docker.github.io,JimGalasyn/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,johnstep/docker.github.io,johnstep/docker.github.io,londoncalling/docker.github.io,londoncalling/docker.github.io,anweiss/docker.github.io,thaJeztah/docker.github.io,johnstep/docker.github.io,JimGalasyn/docker.github.io,anweiss/docker.github.io,docker/docker.github.io,johnstep/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,JimGalasyn/docker.github.io,docker/docker.github.io,johnstep/docker.github.io,docker/docker.github.io,anweiss/docker.github.io,anweiss/docker.github.io,JimGalasyn/docker.github.io,thaJeztah/docker.github.io,JimGalasyn/docker.github.io,anweiss/docker.github.io