Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint, idempotentHint, openWorldHint=false and destructiveHint=false. The description adds value beyond that by disclosing the exact composition of each result (explanation, section URL with anchor, cited docs links) and a staleness caveat about verifying syntax against current docs.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.