metrics: update metric description for `sys.cpu.host.combined.percent-normalized`

**Is your feature request related to a problem? Please describe.**
The current Prometheus metric description for `sys.cpu.host.combined.percent-normalized` has caused some confusion with ambiguous terminology.

Currently, the docs team is working (https://github.com/cockroachdb/docs/pull/17465) to update our docs with better descriptions, which will be reflected in DB Console metrics pages (https://github.com/cockroachdb/cockroach/issues/107819).

In addition to this work, we need to update the canonical metric description itself in CRDB where its originally defined.

**Describe the solution you'd like**
Once the other noted issues above have found alignment on a new description, update the metric metadata for `sys.cpu.host.combined.percent-normalized` to also reflect the new description.

Jira issue: CRDB-30413