Merge #15254: Trivial: fixup a few doxygen comments

Summary:
70e7cee96028889ab709fa3c479225e584fcb77f Trivial: Doxygenize existing CBufferedFile and VectorReader comments (Ben Woosley)
9431e1b91598fc255234ede10c22208ad18685cd Trivial: fixup a few doxygen comments (Ben Woosley)

Pull request description:

These were not declared properly, so their results are not properly
processed. E.g.:
https://dev.visucore.com/bitcoin/doxygen/rpcdump_8cpp.html#a994c8748aaa60fbb78009ff8a0638dea
https://dev.visucore.com/bitcoin/doxygen/coins_8cpp.html#aa03af24ef3570144b045f4fca7a0d603
https://dev.visucore.com/bitcoin/doxygen/wallet_2wallet_8cpp.html#a5c2a7725ff8796f03471f844ecded3d9

> A third alternative is to use a block of at least two C++ comment lines, where each line starts with an additional slash or an exclamation mark.

http://www.doxygen.nl/manual/docblocks.html

Tree-SHA512: c13fd48ac78f25e51b1281385747e8be4bd6e27e0a0f8704608aa5c66c16715c639f1cf27018b1bf05fc2eaed6c7b9be05a68365ffe1d88dd3f400d453b7bead

Backport of Core PR15254

Review notes:
VerifyWallets() has already been removed.
Changes to wallet/rcpdump.cpp were made in D6140.

Test Plan:

ninja

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6475