Make a note about a potentially confusing behavior with `BaseIndex::m_synced`;
if the user starts bitcoind with an empty datadir and an index enabled,
BaseIndex will consider itself synced (as a degenerate case). This affects
how indices are built during IBD (relying solely on BlockConnected signals vs.
using ThreadSync()).
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/22485 | core#22485]]