qapi: expand docs for SEV commands

qapi: expand docs for SEV commands

This gives some more context about the behaviour of the commands in
unsupported guest configuration or platform scenarios.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-ID: <20250522190542.588267-3-pierrick.bouvier@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Tweak query-sev doc, turn error descriptions into Errors sections,
delate a stray #, normalize whitespace, wrap lines]