Clarify the Image! docs.

To clarify something I found hard to understand.

Author: nnethercote

crates/spirv-std/macros/src/lib.rs

@@ -113,7 +113,8 @@ use std::fmt::Write;
 ///   when set the image format is unknown.
 ///   Accepted values: `f32`, `f64`, `u8`, `u16`, `u32`, `u64`, `i8`, `i16`, `i32`, `i64`.
 /// - `format` — The image format of the image, mutually exclusive with `type`.
-///   Accepted values: Snake case versions of [`ImageFormat`].
+///   Accepted values: Snake case versions of [`ImageFormat`] variants, e.g. `rgba32f`,
+///   `rgba8_snorm`.
 /// - `sampled` — Whether it is known that the image will be used with a sampler.
 ///   Accepted values: `true` or `false`. Default: `unknown`.
 /// - `multisampled` — Whether the image contains multisampled content.