Today the only example of calling the cluster allocation explain API above the
fold is the bare `GET /_cluster/allocation/explain` which kind of works but is
not usually what the user wants. This commit changes the docs so that we open
with an example showing how we usually expect it to be called. This will make
it clearer that you should normally specify exactly for which shard you want an
explanation. It also tidies up a few other wrinkles in these docs.
Co-authored-by: James Rodewig <40268737+jrodewig@users.noreply.github.com>
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