Make field warnings more approachable

[w33ble]

Field warning messages in Discover are pretty technical and may not make sense to a lot of users. I believe the solution was to use a non-technical warning here, and link to a longer, more technical explanation, or even to the ES docs.

[loren]

Not sure if this would be a separate issue or not, but I would prefer a config option to disable these warnings altogether. If we roll out Kibana to 000's of users via Shield, we will get 000's of support emails asking about this warning.

[lukasolson]

These error messages aren't shown any more.

[lukasolson]

These error messages were more pertinent when field data was turned on by default. Now that it's an opt-in option, they aren't as important.