[Lens] Better description of what color continuity is in dynamic coloring

[dej611]

Describe the feature:

The current implementation of dynamic coloring in Lens datatable computes the gradient and extends the coloring out on the extremes to all values outside the computed range.
This concept is not super clear as reported also by @mbondyra . Perhaps add some help text may clear out the concept to users.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)