Fix wait_for_completion option description and default setting docs

[epugh]

Description

The documentation for the wait_for_completion turns out to be wrong, the default value is true. However, more importantly, it's not a great doc... I didn't get what it did at first till a colleague pointed it out and then someone on #general clarified it for me!

I did a quick search for wait_for_completion across docs, and stylisticially it's documented in a few ways. For my fix, I took the descirption used in delete-by-query.md.

I did NOT try to fix the other places where maybe the option description could be improved or standardized....

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

Version

2.16

Checklist

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[Naarcha-AWS]

@epugh: Let me know once you've implemented the changes and we'll get this merged.