Change allocation explain message

[DiannaHohensee]

Reduce the amount of text returned when no shard is requested and all
shards are assigned. Less text is easier to digest and hopefully will
not be skimmed.

[elasticsearchmachine]

Pinging @elastic/es-distributed-coordination (Team:Distributed Coordination)

[DaveCTurner]

Ok by me, but I wonder, maybe we should just reverse the order so the next-steps instruction is first and then the context is second. Something like this:

                    Specify an assigned shard in the request body to explain its allocation. No shard was specified in the request which \
                    means to explain the allocation of a randomly-chosen unassigned shard, but there are no unassigned shards in this \
                    cluster. See %s for more information.""", ReferenceDocs.ALLOCATION_EXPLAIN_API));

[DiannaHohensee]

I wonder, maybe we should just reverse the order so the next-steps instruction is first and then the context is second. Something like this:...

That still keeps it relatively wordy. I'd actually like to move explanation (no pun intended 🙄) of the default behavior when no shard is specified to the public docs. Then the response is like 'hey, no unassigned like you wanted, but you can do this other thing'.

I stole some of your new text, though: even more concise.

[DaveCTurner]

Seems worth a shot. It's basically a losing battle when folks don't read the message anyway...