Merge #17719: Document better -keypool as a look-ahead safety mechanism

f41d58966995fe69df433fa684117fae74a56e66 Document better -keypool as a look-ahead safety mechanism (Antoine Riard)

Pull request description:

  If after a backup, an address is issued beyond the initial
  keypool range and none of the addresses in this range
  is seen onchain, if a wallet is restored from backup, even in
  case of rescan, funds may be loss due to the look-ahead
  buffer not being incremented and so restored wallet not detecting
  onchain out-of-range address as derived from its seed.

  This scenario is theoretically unavoidable due to the requirement
  of the keypool to have a max size. However, given the default
  keypool size, this is unlikely. Document better keypool size
  implications to avoid user setting a too low value.

  While reviewing #17681, it took me a while to figure out the safety implications of keypool, I find it would be better to document this a bit farther to avoid users shooting themselves in the foot. For further context & discussion, see https://github.com/bitcoin/bitcoin/pull/17681#issuecomment-563613452

ACKs for top commit:
  ryanofsky:
    Code review ACK f41d58966995fe69df433fa684117fae74a56e66. Just "Warning:" prefix added since the last review
  jonatack:
    ACK f41d58966995fe69df433fa684117fae74a56e66 code review and build/test. The added `Warning:` since last review is a good addition.

Tree-SHA512: d3d0ee88fcdfc5c8841a2bd4bada0e4eeb412a0dce5054e5fb023643c2fa57206a0f3efb06890c245528dc4431413ed2fd5645b9319d26245d044c490b7f0db0

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