docs: clarify QR-code verification flow and generateQRCode preconditions (#5412)
Document that startVerification() is for SAS (emoji) verification only and
rejects QR-code methods, and enumerate the conditions under which
generateQRCode() returns data versus undefined (phase Ready, other party
supports m.qr_code.scan.v1, and local cross-signing keys available). Add a
usage note to scanQRCode(). Comments only; no behavioural change.
Fixes #5092
Signed-off-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>