docs: clarify TextureSystem::create use of imagecache when shared=true

Signed-off-by: Larry Gritz <lg@larrygritz.com>

src/include/OpenImageIO/texture.h

@@ -384,7 +384,9 @@ class OIIO_API TextureSystem {
     ///     freeing the ImageCache after the TextureSystem is destroyed.  If
     ///     `shared` is `false` and `imagecache` is empty, then a custom
     ///     ImageCache will be created, owned by the TextureSystem, and
-    ///     automatically freed when the TS destroys.
+    ///     automatically freed when the TS destroys. If `shared` is true,
+    ///     this parameter will not be used, since the global shared
+    ///     TextureSystem uses the global shared ImageCache.
     ///
     /// @returns
     ///     A shared pointer to a TextureSystem which will be destroyed only