Merge #10685: Clarify CCoinsViewMemPool documentation.

Summary:
381b8fc36 Clarify CCoinsViewMemPool documentation. (Matt Corallo)

Tree-SHA512: 0d1520595a03ea4038b4119871d79dfc28a50f42ac7667b30a583805d4fe4b0480a6912b9b4f7660c7223778ef85f2f9d1870551383a35c5e8c88cd2935f59d3

Backport of Core PR10685
https://github.com/bitcoin/bitcoin/pull/10685/files

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, markblundeberg, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3622