More concerns with storage classes

I was reading through the chapter on Storage Classes (https://github.com/KhronosGroup/SPIRV-Guide/blob/main/chapters/storage_class.md), and I have questions about `Uniform` vs `UniformConstant`. Specifically, the chapter claims that:

> `Uniform` is read-only

but I haven't found that to be totally accurate, nor do I see support for that assertion in the SPIR-V spec (which instead calls `UniformConstant` data read-only).

Consider this GLSL shader:

```
#version 450
layout (local_size_x = 1, local_size_y = 1, local_size_z = 1) in;

struct Particle {
float position;
float velocity;
};

layout(std140, binding = 1) buffer ParticleIO {
Particle particles[];
};

void main() {
const float location_bound = 100;
float result = particles[gl_GlobalInvocationID.x].position
+ particles[gl_GlobalInvocationID.x].velocity;
particles[gl_GlobalInvocationID.x].position = result;
}
```

When it is translated to SPIR-V 1.2, we see:
`%_ = OpVariable %_ptr_Uniform_ParticleIO Uniform`

and later:
```
%74 = OpAccessChain %_ptr_Uniform_float %_ %int_0 %71 %int_0
OpStore %74 %73
```

Thus, the uniform variable is *not* read-only (unless the `glslang` translation was incorrect).

This begs a follow-up question: why not use a `UniformConstant` in this case? The chapter says:

> `UniformConstant` can be viewed as a "opaque handle" to images, samples, raytracing acceleration structures, etc variables.

It seems like this is a case which belongs neatly with that enumeration.

I am having a hard time coming up with a solid mental model / abstraction for the two storage classes. The SPIR-V spec says that `UniformConstant` is read-only, but that is complicated by the fact that only the handle itself is read-only. This guide says that `Uniform` is read-only, but this issue appears to be another complication to that rule. If both `Uniform` and `UniformConstant` are read-only handles, what differentiates them?

Start with chapters/storage_class.md, then compare its claims with the SPIR-V specification and the shown GLSL-to-SPIR-V output from glslang. Use the Uniform and UniformConstant examples in the issue to determine the missing distinction, then update the chapter so its explanation is accurate and gives readers a usable mental model.

Written by the indexing model from the issue text.