[ML] Add docs about when the Single Metric Viewer will and won't work

[droberts195]

Currently the Single Metric Viewer only works in limited situations.

The logic to decide whether it will or won't work is very complex. As far as I can see there are no public docs that explain in plain English when it is and isn't expected to work.

All the different situations result in the message, "it is not a viewable time series job". This message does not say why the particular job is not viewable in the single metric viewer, which can be frustrating for users and makes it very hard for anyone doing troubleshooting to explain why either.

While the single metric viewer only works in certain situations there should be docs to say what conditions need to be met for it to work, and giving examples of the sorts of job configurations that will stop the single metric viewer from working.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[szabosteve]

Related PR: elastic/stack-docs#2672

[szabosteve]

Resolved via elastic/stack-docs#2672.