[backport#16757] doc: CChainState return values

Summary:
fa912a8ad5a94cd2bdc149400b1befb346621f03 doc: move-only ActivateBestChain doxygen comment to header (MarcoFalke)
fa99efd054c57cd6717391f9ae8ce32b06986ff8 doc: ActivateBestChainStep return value (MarcoFalke)

Pull request description:

It will always return true, unless a system error such as #15305 occurred

ACKs for top commit:

laanwj:
  ACK fa912a8ad5a94cd2bdc149400b1befb346621f03

Backport of Core PR16757

Test Plan:

read the comments

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7958