doc: BaseIndex sync behavior with empty datadir

Summary:
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 core#22485

Depends on D17702

Test Plan: proof read

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17703