Remove ForceUnroll attribute from link-time specialization documentation (#8225)

The link-time specialization documentation contained an incorrect
example that used `[ForceUnroll]` with a link-time type method call,
which would cause a compilation error. The issue was that
`[ForceUnroll]` requires loop bounds to be known at compile time, but
`sampler.getSampleCount()` is a method call that returns a value at
runtime.

**Problem:**
The documentation example showed:
```csharp
Sampler sampler;
[ForceUnroll]
for (int i = 0; i < sampler.getSampleCount(); i++)
    output[tid] += sampler.sample(i);
```

This would fail with error: `loop does not terminate within the limited
number of iterations, unrolling is aborted.`

**Solution:**
Removed the `[ForceUnroll]` attribute entirely, leaving a simple loop:
```csharp
Sampler sampler;
for (int i = 0; i < sampler.getSampleCount(); i++)
    output[tid] += sampler.sample(i);
```

Since the loop bounds come from a runtime method call, there's no way
for the loop to be unrolled regardless of the directive used, so the
simplest solution is to remove the unroll attribute completely.

- [x] Remove ForceUnroll attribute from documentation example
- [x] Remove explanatory note about unroll vs ForceUnroll
- [x] Remove test cases for the removed functionality
- [x] Fix missing closing backticks in code block

Fixes #8161.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: bmillsNV <163073245+bmillsNV@users.noreply.github.com>
Co-authored-by: expipiplus1 <857308+expipiplus1@users.noreply.github.com>

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