docs: document hostnames in cluster file and coordinators command

[VanshBhardwaj24]

The docs still say that the cluster file lists IP addresses "(not hostnames)", and that the coordinators command and the coordinators/processes special key take IP:port pairs. Hostnames are supported:

• ClusterConnectionString parses each entry with Hostname::isHostname, so a list can mix hostnames and IP addresses (fdbclient/MonitorLeader.cpp), and Hostname::parse accepts an optional :tls suffix (flow/Hostname.cpp).
• The coordinators command and the special key accept HOSTNAME:PORT (fdbcli/CoordinatorsCommand.cpp, fdbclient/SpecialKeySpace.cpp). changeQuorumChecker resolves the hostnames and fails with COORDINATOR_UNREACHABLE if any of them does not resolve, and reading the special key returns the resolved addresses (fdbclient/ManagementAPI.cpp, fdbclient/SpecialKeySpace.cpp).
• coordinators auto returns network addresses (AutoQuorumChange in fdbclient/ManagementAPI.cpp), so if it changes the coordinators the cluster file lists IP addresses instead of hostnames. guide-common.rst.inc already says it selects processes by IP address.
• The 7.1.4 release notes list "Added hostname DNS resolution logic for cluster connection string" (Check in the new Hostname logic (release-7.1). #6998), and the 7.1.8 notes list "Fixed an issue that coordinatorsKey does not store DNS names" (coordinatorsKey should not always store IP addresses (release-7.1). #7203).
• The /fdbclient/MonitorLeader/parseConnectionString/hostnames unit test covers hostname-only connection strings, the :tls suffix, and rejection of duplicate hostnames.

This updates administration.rst, the shared cluster-file rule in guide-common.rst.inc, command-line-interface.rst and special-keys.rst to describe the HOSTNAME:PORT form and these caveats. The new text only states what the code and tests above show.

Not changed here: the built-in fdbcli help text for coordinators, architecture.rst and kv-architecture.rst still mention only IP:port. Those can go in a follow-up.

Validation: the Sphinx 6.2.1 HTML build passes with warnings treated as errors, using the same sphinx-build -W invocation as documentation/CMakeLists.txt. git diff --check is clean. The unit tests and fdbcli were not built or run.

Docs-only change; no code or behavior is affected.

Related to #3469.

[ShreeChaturvedi]

Thanks for this! One correction before merging: the new text says hostnames are kept as written, but coordinators auto writes IP addresses. If auto changes the coordinators, the hostnames in the cluster file are replaced. On a local cluster:

fdb> coordinators coord1.fdbtest:4700
  cluster file: ...@coord1.fdbtest:4700
fdb> configure double
fdb> coordinators auto
  cluster file: ...@127.0.0.1:4701,127.0.0.1:4702,127.0.0.1:4703

I've suggested wording inline. A few smaller things readers might run into:

• Running coordinators (or reading the special key) shows IPs even when the file has hostnames, so it looks like the change didn't work.
• Every hostname has to resolve when you run coordinators, or the command fails.
• Order isn't kept: IPs go first, then hostnames.

Fine for this PR: architecture.rst, kv-architecture.rst, and help coordinators still say IP:PORT only.

[VanshBhardwaj24]

Thanks @ShreeChaturvedi, that was a real miss on my part. I checked each point against the code and pushed an update:

• coordinators auto returns NetworkAddresses (AutoQuorumChange::getDesiredCoordinators), so it does replace hostnames with IPs. I used your wording in administration.rst and the coordinators CLI section.
• changeQuorumChecker returns COORDINATOR_UNREACHABLE when a hostname doesn't resolve, and coordinatorsGetRangeActor returns the resolved addresses, so I added both points, one to the CLI text and one to the special-keys text.
• I dropped the character list. The regex in flow/Hostname.cpp rejects things like a leading or doubled dot, so the list over-claimed.

I left architecture.rst, kv-architecture.rst and the help coordinators text for a follow-up, as you suggested. I also updated the PR description to match.