[DOCS] Add missing description for `transport_client` reserved role

[slobodanadamovic]

This PR adds missing role description for the transport_client role, and a test to enforce that all reserved roles are described. The description also serves as self-documentation for roles, thus it is reasonable to make this a requirement for all reserved roles.

Relates to https://github.com/elastic/elasticsearch/pull/108422, which included descriptions for other reserved roles.

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

Pinging @elastic/es-security (Team:Security)

[slobodanadamovic]

The role is deprecated, so I would add that to the description as well.

I think we still need to deprecate it. I'll leave the description as is and handle deprecation in a followup PR.