[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