docs: tag every documented method's parameters and return

[tas50]

yard stats already reported 100% documented, which hid the actual gap: every method had a docstring, but twelve of them documented only what the method does and left the signature untagged.

# Works out the address Test Kitchen should connect to, allocating a
# public address and forwarding the transport's port when asked to.
def hostname_for(state, server_info)

Prose, no @param, no @return. YARD counts that as documented — the metric measures presence, not completeness — so the driver looked fully covered while a reader still had to go read the body to learn that state is mutated and what comes back.

Twelve methods, all of them either private helpers or initialize:

file	methods
driver/cloudstack.rb	deploy_instance, hostname_for, apply_credentials
cloudstack/client.rb	initialize, job_error
cloudstack/credentials.rb	initialize
cloudstack/networking.rb	initialize, create_port_forward, teardown, tolerating_missing
cloudstack/server_options.rb	initialize, truncate_to_budget

The tags say which state keys each method adds, since that is the part worth knowing and the part the prose kept implicit — deploy_instance gains server_id, hostname_for gains ipaddressid and forwardingruleid, create_port_forward gains forwardingruleid. tolerating_missing also gets the @yield and the @raise it re-raises on.

Documentation only. No behaviour changed.

Verification

• every method taking arguments now carries @param (checked by script: 0 remaining)
• yard stats — 44 methods, 0 undocumented, 100%
• rake test — 63 examples, 0 failures
• cookstyle --chefstyle — 9 files, no offenses