Clarify scope of pipeline creation

[alan-baker]

Fixes #4648

• Clarify that pipeline-creation only includes the code encompassed by the shader interface of the entry point being compiled
• Clarify that value substitution occurs before override-expression evaluation
  • Add examples of potential errors
• Clarify pipeline-creation errors only occur for the portion of the module being compiled

[alan-baker]

I made this draft until @jimblandy checks his implementation and signs off on #4648.

[github-actions[bot]]

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