Add descriptions to GlslangToSpv.h

Inside https://github.com/KhronosGroup/glslang/blob/main/SPIRV/GlslangToSpv.h there is

```c++
struct SpvOptions {
bool generateDebugInfo {false};
bool stripDebugInfo {false};
bool disableOptimizer {true};
bool optimizeSize {false};
bool disassemble {false};
bool validate {false};
bool emitNonSemanticShaderDebugInfo {false};
bool emitNonSemanticShaderDebugSource{ false };
bool compileOnly{false};
bool optimizerAllowExpandedIDBound{false};
};
```

and no good way without reading the code or experimenting to know what these do.

Some simple comments on these to explain either what they do, or even the equivalent command line argument they emulate