[backport#15777] [docs] Add doxygen comment for CKeyPool

Summary:
[docs] Add doxygen comment for CReserveKey

https://github.com/bitcoin/bitcoin/pull/15777/commits/37796b2dd49772b17ff39a1a71b73f6d2248ac6d
https://github.com/bitcoin/bitcoin/pull/15777/commits/f1a77b0c5176306ca9f6f30211e32d3502ed4281

Concludes backport of Core PR15777

Test Plan:

ninja

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6350