Expand and clarify docs on persistent tasks

[timholy]

While helping someone on slack I noticed these docs were a bit less clear than they could be. Here's my attempt at improving them. CC @NHDaly.

[NHDaly]

👍

[NHDaly]

❤️ I love it. Thank you!

[lgoettgens]

Thanks for the PR. If you and @NHDaly are happy with the changes, there is no further action required from your side. I'll go ahead with updating the changelog and preparing a release with this.
If this PR is still open in a few days, I might have forgotten it. Feel free to ping me then.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.90%. Comparing base (3499e74) to head (8abc65f).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #297      +/-   ##
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+ Coverage   74.77%   74.90%   +0.13%     
==========================================
  Files          11       11              
  Lines         761      761              
==========================================
+ Hits          569      570       +1     
+ Misses        192      191       -1     

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[timholy]

Thanks for the review and for merging it! And of course especially for your ongoing maintenance, @lgoettgens, of this crucial package!