Update UI to make it clearer what reindex does and does not do

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Problem

We have received many customer reports of them having problems running the reindex via the UI. In almost all cases we've seen there was actually no reason for them to be using the reindex option in the UI. They regularly are assuming it is going to fix data issues or is necessary for other reasons. Usually it makes the root cause worse.

Solution

Update the UI to explain that it cannot fix missing data and should only be used when you know there is some index settings you are trying to update. Also point the user to the rake task rake gitlab:elastic:index if their intention is to recreate the index from scratch to correct data issues.