bpo-22702: Clarify documentation of str.join & bytes.join (GH-156) (GH-1897)

bpo-22702: Clarify documentation of str.join & bytes.join (GH-156) (GH-1897)

The "iterable iterable" phrasing created confusion between the term
reference and the parameter name.

This simplifies the phrasing to just use the parameter name
without linking directly to the term definition.
(cherry picked from commit 08e2f355d04d3cbea5751ce1275306ee3f569b32)