Update some links and clean up comments

Also, bring back appendix D math shortcut

`fy ** 3 if l > KE else l / KAPPA` is functionally equivalent to
`fy ** 3 if fy > EPSILON3 else (116 * fy - 16) / KAPPA`, it just saves
us from doing redundant math and is what is recommended in appendix D
and the example used by Bruce Lindbloom.

I admit the second approach is less confusing, but the first is more
efficient. We briefly considered the more intuitive approach over the
efficient approach and have decided we will stick with efficient.

coloraide/spaces/lab/__init__.py

@@ -1,4 +1,9 @@
-"""Lab class."""
+"""
+Lab class.
+
+https://ia802802.us.archive.org/23/items/gov.law.cie.15.2004/cie.15.2004.pdf
+http://www.brucelindbloom.com/Eqn_Lab_to_XYZ.html
+"""
 from __future__ import annotations
 from ...spaces import Space, Labish
 from ...cat import WHITES
@@ -15,14 +20,7 @@
 
 
 def lab_to_xyz(lab: Vector, white: VectorLike) -> Vector:
-    """
-    Convert Lab to D50-adapted XYZ.
-
-    http://www.brucelindbloom.com/Eqn_Lab_to_XYZ.html
-
-    While the derivation is different than the specification, the results are the same as Appendix D:
-    https://www.cdvplus.cz/file/3-publikace-cie15-2004/
-    """
+    """Convert CIE Lab to XYZ using the reference white."""
 
     l, a, b = lab
 
@@ -34,7 +32,7 @@ def lab_to_xyz(lab: Vector, white: VectorLike) -> Vector:
     # compute `xyz`
     xyz = [
         fx ** 3 if fx > EPSILON3 else (116 * fx - 16) / KAPPA,
-        fy ** 3 if fy > EPSILON3 else (116 * fy - 16) / KAPPA,
+        fy ** 3 if l > KE else l / KAPPA,
         fz ** 3 if fz > EPSILON3 else (116 * fz - 16) / KAPPA
     ]
 
@@ -43,14 +41,7 @@ def lab_to_xyz(lab: Vector, white: VectorLike) -> Vector:
 
 
 def xyz_to_lab(xyz: Vector, white: VectorLike) -> Vector:
-    """
-    Assuming XYZ is relative to D50, convert to CIELab from CIE standard.
-
-    http://www.brucelindbloom.com/Eqn_XYZ_to_Lab.html
-
-    While the derivation is different than the specification, the results are the same:
-    https://www.cdvplus.cz/file/3-publikace-cie15-2004/
-    """
+    """Convert XYZ to CIE Lab using the reference white."""
 
     # compute `xyz`, which is XYZ scaled relative to reference white
     xyz = alg.divide(xyz, white, dims=alg.D1)