Link features, constraints for shader stages

[dj2]

In 9.1. Shader Stages we say:

Each shader stage has its own set of features and constraints, described elsewhere.

It would be useful if the described elsewhere were links to the relevant sections of the specification.

[dneto0]

Also, think about pulling constraints together. E.g.

• vertex shader must have a builtin position output, but that rule is in Function restrictions section
• compute shader must not have user-defined inputs or outputs; that rule is in 9.3.1.2