DOC: server: document 'set server name' CLI command

DOC: server: document 'set server name' CLI command

Document the new 'set server <b>/<s> name <newname>' CLI command in
management.txt.

The documentation states the two preconditions that gate the operation
(server must be in maintenance, server's name must not be statically
referenced via use-server / track / ARGT_SRV), notes that the command
is not gated by a per-backend opt-in directive (parity with 'add
server' / 'del server'), and mentions the EVENT_HDL_SUB_SERVER_NAME
event published on successful rename so Lua and other event consumers
know to subscribe to it.