docs: explain smoothstep usage in baseStrokeShader

[jjnawaaz]

Addresses #8442

Changes

• Added documentation explaining the GLSL smoothstep(edge0, edge1, x) function
• Included a short conceptual explanation of how smoothstep interpolates values
• Documented its common usage for soft edges and anti-aliased stroke effects in the baseStrokeShader reference section

Screenshots of the change

Not applicable (documentation-only change).

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated

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