DOC: management: document 'clear counters server'

DOC: management: document 'clear counters server'

Add a section for the new 'clear counters server <backend>/<server>
[force]' CLI command in doc/management.txt, alongside 'clear counters'
and 'clear counters all'.

The entry documents the admin-only permission level (it clears
accumulated counters, like 'clear counters all') and the fact that both
accumulated counters and max values are reset, along with the clearable
module-registered extra counters, while the server's runtime state
(address, weight, admin state) and health checks are left untouched. It
describes the optional 'force' argument required to clear counters
stored in a shared-memory stats file, and calls out the Kubernetes
slot-recycle use case that motivated the command (per-entity
attribution after 'set server <b>/<s> name').