Update shadermodel.md

[thetyl]

What does this PR do? Update shadermodel documentation to replace auto-generated text.

Thanks for the contribution! Please provide a concise description of the problem this request solves.

How does this PR change Premake's behavior? Modifies the Premake documentation.

Are there any breaking changes? Will any existing behavior change? No.

Anything else we should know? No.

Add any other context about your changes here.

Did you check all the boxes?

• [CHECK ] Focus on a single fix or feature; remove any unrelated formatting or code changes
• [CHECK ] Add unit tests showing fix or feature works; all tests pass
• [CHECK ] Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• [CHECK ] Follow our coding conventions
• [CHECK ] Minimize the number of commits
• [ CHECK] Align documentation to your changes

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[tommitytom]

Is there an example of this shader stuff anywhere? And does it only support HLSL?

[thetyl]

Yes it only supports HLSL. There is an example usage included in this change.

[thetyl]

@nickclark2016 Could you please approve this PR?

[nickclark2016]

Just saw the message, the last several weeks have been hectic for me. Still waiting on you to address that last comment, then I'll go ahead and merge.

[KyrietS]

@TylerDahl ?

[thetyl]

@KyrietS Addressed the final issue for line 39.

[KyrietS]

Let's wait for @nickclark2016's approval