Correct refraction ratio definition

docs/api/materials/MeshBasicMaterial.html

@@ -117,7 +117,11 @@
 		</div>
 
 		<h3>[property:Float refractionRatio]</h3>
-		<div>The index of refraction for an environment map using [page:Textures THREE.CubeRefractionMapping]. Default is *0.98*.</div>
+		<div>
+			The index of refraction (IOR) of air (approximately 1) divided by the index of refraction of the material.
+			It is used with environment mapping modes [page:Textures THREE.CubeRefractionMapping] and [page:Textures THREE.EquirectangularRefractionMapping].
+			The refraction ratio should not exceed 1. Default is *0.98*.
+		</div>
 
 		<h3>[property:Boolean skinning]</h3>
 		<div>Define whether the material uses skinning. Default is false.</div>

docs/api/materials/MeshLambertMaterial.html

@@ -143,7 +143,11 @@
 		<div>How much the environment map affects the surface; also see [page:.combine].</div>
 
 		<h3>[property:Float refractionRatio]</h3>
-		<div>The index of refraction for an environment map using [page:Textures THREE.CubeRefractionMapping]. Default is *0.98*.</div>
+		<div>
+			The index of refraction (IOR) of air (approximately 1) divided by the index of refraction of the material.
+			It is used with environment mapping modes [page:Textures THREE.CubeRefractionMapping] and [page:Textures THREE.EquirectangularRefractionMapping].
+			The refraction ratio should not exceed 1. Default is *0.98*.
+		</div>
 
 		<h3>[property:Boolean skinning]</h3>
 		<div>Define whether the material uses skinning. Default is false.</div>

docs/api/materials/MeshPhongMaterial.html

@@ -192,7 +192,11 @@
 		</div>
 
 		<h3>[property:Float refractionRatio]</h3>
-		<div>The index of refraction for an environment map using [page:Textures THREE.CubeRefractionMapping]. Default is *0.98*.</div>
+		<div>
+			The index of refraction (IOR) of air (approximately 1) divided by the index of refraction of the material.
+			It is used with environment mapping modes [page:Textures THREE.CubeRefractionMapping] and [page:Textures THREE.EquirectangularRefractionMapping].
+			The refraction ratio should not exceed 1. Default is *0.98*.
+		</div>
 
 		<h3>[property:Float shininess]</h3>
 		<div>How shiny the [page:.specular] highlight is; a higher value gives a sharper highlight. Default is *30*.</div>

docs/api/materials/MeshStandardMaterial.html

@@ -226,7 +226,11 @@
 		</div>
 
 		<h3>[property:Float refractionRatio]</h3>
-		<div>The index of refraction for an environment map using [page:Textures THREE.CubeRefractionMapping]. Default is *0.98*.</div>
+		<div>
+			The index of refraction (IOR) of air (approximately 1) divided by the index of refraction of the material.
+			It is used with environment mapping modes [page:Textures THREE.CubeRefractionMapping] and [page:Textures THREE.EquirectangularRefractionMapping].
+			The refraction ratio should not exceed 1. Default is *0.98*.
+		</div>
 
 		<h3>[property:Float roughness]</h3>
 		<div>