doc: update description of network-manager service setting (#774)
doc: update description of network-manager service setting (#774)

As #774 is resolved, it is no longer true that NetworkManger in
non-NetVM will cause network problems.

woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin