[docs] Improve commenting in coins.cpp|h

Summary:
Remove references to 'pruned' coins, which don't exist since the move
to per-txout coins db.

This is a backport of Core PR18410 [1/3]
https://github.com/bitcoin/bitcoin/pull/18410/commits/c205979031ff4e8e32a5f05bae813405f233fccd

Test Plan: proof-reading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8953