Document preferred plural: indexes vs indices

What does this MR do?

In some work for groupdatabase, I wondered if we were consistent in using indexes vs indices. As it turns out, we aren't -- and we can't be consistent:

• The database world has settled on indexes. https://www.postgresql.org/docs/15/glossary.html doesn't have an explicit statement preferring indexes but the documentation consistently uses it instead of indices.
• ElasticSearch explicitly uses indices: https://brand.elastic.co/302f66895/p/194a3b-writing-style-guide/t/60bf66

I'll have to think about how to implement this as a test, because this would be a good one to catch, but for now getting the explanation in the word list would be good.

Related issues

Closes Plural of index: indexes, or indices? (#411930 - closed)

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