Merge pull request #20306 from gkjohnson/unroll-docs

Docs: Clarify loop unroll behavior

docs/api/en/materials/ShaderMaterial.html

@@ -54,6 +54,9 @@
 					<li>
 						The loop variable has to be *i*.
 					</li>
+					<li>
+						The value *UNROLLED_LOOP_INDEX* will be replaced with the explicity value of *i* for the given iteration and can be used in preprocessor statements.
+					</li>
 				</ul>
 				<code>
 		#pragma unroll_loop_start

docs/api/zh/materials/ShaderMaterial.html

@@ -43,6 +43,9 @@
 					<li>
 						循环变量必须是*i*。
 					</li>
+					<li>
+						The value *UNROLLED_LOOP_INDEX* will be replaced with the explicity value of *i* for the given iteration and can be used in preprocessor statements.
+					</li>
 				</ul>
 				<code>
 		#pragma unroll_loop_start