[DOCS] Add descriptions to the spec files for internal and blocked APIs

[szabosteve]

Tracking issue for track doc updates to specification files. To prevent duplicated work, leave your name beside a namespace/API you start working on.

Related to #964.

Serverless plan: https://docs.google.com/spreadsheets/d/1xFrpFP1dARey8A1QkAoC7JJCGhAZFexMLpRTeY9PAos/

• autoscaling BLOCKED
• ccr BLOCKED
• dangling_indices BLOCKED
• features BLOCKED
• fleet INTERNAL
• ilm BLOCKED
• license BLOCKED (GET license is addressed)
• migration BLOCKED
• monitoring BLOCKED
• searchable_snapshots INTERNAL
• security most of the endpoints are addressed, the rest is INTERNAL
• shutdown BLOCKED
• slm BLOCKED
• snapshot INTERNAL
• ssl INTERNAL
• watcher not working on serverless at least for now
• xpack INTERNAL