Explain area of get_ztf_footprint_corners() polygon

This polygon is smaller than the spatial extent of the true ZTF
field of view, but has approximately the same area because the real
ZTF field of view has chip gaps.

For the real ZTF footprint, use the region file
https://github.com/skyportal/skyportal/blob/main/data/ZTF_Region.reg.

healpix_alchemy/tests/benchmarks/data.py

@@ -25,7 +25,17 @@
 
 
 def get_ztf_footprint_corners():
-    """Return the corner offsets of the ZTF footprint."""
+    """Return the corner offsets of the ZTF footprint.
+
+    Notes
+    -----
+    This polygon is smaller than the spatial extent of the true ZTF field of
+    view, but has approximately the same area because the real ZTF field of
+    view has chip gaps.
+
+    For the real ZTF footprint, use the region file
+    https://github.com/skyportal/skyportal/blob/main/data/ZTF_Region.reg.
+    """
     x = 6.86 / 2
     return [-x, +x, +x, -x] * u.deg, [-x, -x, +x, +x] * u.deg