Include the "three important characteristics" in registry-specific documentation

[shakuzen]

See https://github.com/micrometer-metrics/micrometer/issues/2469#issuecomment-788760788 for background.

We link to our documentation on "rate aggregation" in the "Supported monitoring systems" section, but we don't mention this information from the registry-specific documentation page. We could improve the registry-specific documentation by mentioning the properties mentioned in the "Supported monitoring systems" section and cross-linking to it. Without this, it can be more easily missed and the published metrics can be surprising to users.