Add max_value property to docs

What does this MR do?

@rpereira2 suggested

On a related note, we should add max_value to the docs on Dashboard YAML properties.

Usage of max_value is described under the topic "Percentile based results", but the table of dashboard yaml properties is a good summary of all properties that our dashboards support, so we should keep it up-to-date.

This MR adds information about max_value property to Dashboard YAML properties section of Prometheus documentation.

Related issues

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