Fix documentation for panel properties

In the docs for panel properties, the type field is of type string in the definition but type enum in the docs. It is complicated because behind the scenes there is an enum of possible values, however, the actual type is string.

We should update the documentation accordingly.

Original Description

The following discussion from !37518 (merged) should be addressed:

• @cablett started a discussion:

  I noticed that this is type "string" whereas the docs say enum - I notice other types say string even though they are also an enum (the type attribute on Link, for example). Should we tweak the docs (that is, https://docs.gitlab.com/ee/operations/metrics/dashboards/yaml.html#metrics-metrics-properties) to be consistent? (follow-on issue, not blocking)