Laurent series docs and doctest coverage improvements

[roed314]

Doctest coverage is part of metaticket #12024.

Both files laurent_series_ring and laurent_series_ring_element lack the slightest introduction, many methods are textually undefined, and where they are, they adhere to the 'Returns' convention.

Component: documentation

Issue created by migration from https://trac.sagemath.org/ticket/12259

[KPanComputes]

comment:1

Basically a memo for me: on sage 5.7.beta2, laurent_series_ring.py has coverage 100%.

[rwst]

Description changed:

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+++ 
@@ -1 +1,3 @@
-Part of metaticket #12024
+Doctest coverage is part of metaticket #12024.
+
+Both files `laurent_series_ring` and `laurent_series_ring_element` lack the slightest introduction, many methods are textually undefined, and where they are, they adhere to the 'Returns' convention.