Add CPU-load caution to `enable_schema_id_validation` property description

What's missing

The in-source description for enable_schema_id_validation does not mention the CPU cost of enabling validation:

https://github.com/redpanda-data/redpanda/blob/7246c9e0ac3421f2bb4eb088dfb9f6a358ce89a8/src/v/config/configuration.cc#L3960-L3966

Enabling schema ID validation triggers decompression of message batches on topics where validation is configured, which can lead to a modest increase in CPU load. Documenting this was an acceptance criterion of CIAINFRA-2397, and the docs property reference carries the caution via a docs-side description override (redpanda-data/docs#1925, originally redpanda-data/docs#1565).

Because the caution exists only in the docs override, every other consumer of the property metadata shows a description without it:

• rpk cluster config output
• the Admin API cluster config schema (/v1/cluster_config/schema)
• Redpanda Console's cluster-config UI

Suggested change

Append a plain-text sentence to the property description in src/v/config/configuration.cc, for example:

Enabling validation causes message batches on configured topics to be decompressed for schema ID checks, which can increase CPU load. Monitor CPU utilization after enabling.

Plain text only (the string is rendered in CLI output), so no docs markup needed.

References

• Docs fix: redpanda-data/docs#1925 (restores the caution in the property reference)
• Original docs work: redpanda-data/docs#1565, DOC-1943
• Internal: CIAINFRA-2397