Bring doctest coverage for rings/power_series* to 100%

[KPanComputes]

This was a part of the meta ticket #12024 but wasn't finished because that goal was reached without this ticket.

Work on this was done in #16199 but not 100%.

Depends on #16199

Component: doctest coverage

Issue created by migration from https://trac.sagemath.org/ticket/14053

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Branch: u/rws/bring_doctest_coverage_for_rings_power_series__to_100_

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Author: Ralf Stephan

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New commits:

3102007

14053: doctests added; typos fixed

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Commit: 3102007

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Description changed:

--- 
+++ 
@@ -1 +1 @@
-This is a part of the meta ticket #12024. 
+This was a part of the meta ticket #12024 but wasn't finished because that goal was reached without this ticket.

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Changed author from Ralf Stephan to none

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Dependencies: #16199