Per mara's review of #1718: the HIVE_FORGE_URL / HIVE_MATRIX_URL comments
carried "network isolation is now unconditional / shared-netns removed"
backstory that doesn't belong on those keys. Trimmed to describe just the
value (the gateway vhost) + the gatewayHost null-guard. The
isolation-removal narrative stays where it's on-topic — the network
module's own deprecated-option descriptions and warning text.
75dbcc2aa2