Review feedback on #7254: the fields kept their netstate-era names after the
type changed to *netevents.Manager or a NetworkWatcher interface.
Rename the Manager-typed fields to netMgr/NetMgr and the interface-typed guard
fields to netWatcher, document that the Manager write methods panic on a nil
receiver while the read methods stay nil-safe, and fix the waiteBeforeRetry
typo.
The relay client keeps its netEvents fields: those hold the NetEvents
interface, so the name already matches the type.
0a03211903