[PR #6313/d149eff6 backport][3.9] docs: Fix statement about setUpAsync() and tearDownAsync()

[patchback]

This is a backport of PR #6313 as merged into master (d149eff).

Since #4732, it's wrong to says that
setUpAsync() and tearDownAsync() do nothing and can be overridden without
calling super.setUpAsync() and super.tearDownAsync().

What do these changes do?

Fix documentation

Are there changes in behavior for the user?

No

Related issue number

None

Checklist

Only docs.

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Remark:
My rst preview wasn't working, I hope there is no rst mistakes.

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@@          Coverage Diff           @@
##             3.9    #6318   +/-   ##
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  Coverage       ?   93.31%           
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  Hits           ?    28340           
  Misses         ?     1852           
  Partials       ?      177           

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