[docs] clarify FDFD-to-FDTD field reconstruction

docs/index.md

@@ -26,6 +26,15 @@ Relevant starting examples:
   mode-weighted, and guided-mode / residual comparisons
 - `examples/fdfd.py` for direct frequency-domain waveguide excitation
 
+For solver equivalence, prefer the phasor-based examples first. They compare
+the extracted `+\omega` content of the FDTD run directly against the FDFD
+solution and are the main accuracy benchmarks in the test suite.
+
+`examples/waveguide_real.py` answers a different, stricter question: how well a
+late raw real snapshot matches `Re(E_\omega e^{i\omega t})` on a monitor plane.
+That diagnostic is useful, but it also includes orthogonal residual structure
+that the phasor comparison intentionally filters out.
+
 The API pages are better read as a toolbox map and derivation reference:
 
 - Use the [FDTD API](api/fdtd.md) for time-domain stepping, CPML, phasor