#21347: use string not list in shell=True example.

Patch by Akira.
Issue #21488: Fix doc of codecs.decode() and codecs.encode(), no keyword support.
Patch written by Brad Aylsworth.
Issue #21488: Oops, the patch for codecs.encode/decode doc was written by
Berker Peksag (already present in Misc/ACKS). The issue was reported by Brad
Aylsworth.