update docs how back channel signout is now performed
update docs how back channel signout is now performed

IdentityServer/IdentityServer4,IdentityServer/IdentityServer4,IdentityServer/IdentityServer4,MienDev/IdentityServer4,MienDev/IdentityServer4,MienDev/IdentityServer4,IdentityServer/IdentityServer4,MienDev/IdentityServer4