docs: improve wait_for_index metrics description (#10717)

Old description of `{plan,worker}.wait_for_index` described the metric
in terms of waiting for a snapshot which has two problems:

1. "Snapshot" is an overloaded term in Nomad and operators can't be
   expected to know which use we're referring to here.
2. The most important thing about the metric is what we're waiting *on*
   before taking a snapshot: the raft index of the object to be
   processed (plan or eval).

The new description tries to cram all of that context into the tiny
space provided.

See #5791 for details about the `wait_for_index` mechanism in general.