The labels that were previously there did not have enough granularity
for cases where we use the blackbox exporter.

The meaning of some of the labels, like `instance` and `alias` was
blurred over time and the information it contains seems to vary in
semantics.

For some alerts, we need to know about the blackbox target as well as
the host that's concerned.

This change to the documentation clarifies that instance only has one
stable semantic and that alias label is always used for containing the
hostname that we as humans would think of when referring to metrics.

To complete the picture, we're adding the `target` label that will
contain the blackbox target, which will untangle the meaning of the
`alias` label.