Pipe type specifier should be explained outside of builtin function section.

Pipe as a type is currently described in the builtin functions section.

We should either move the description of pipe type into a section about other types or we could add a separate section like we did for address spaces qualifier or access qualifiers.

Locate where the pipe type is currently described in the builtin functions section, then review the surrounding sections for other types, address spaces qualifiers, and access qualifiers. Decide whether moving the description or adding a separate section best matches the existing organization, and confirm the pipe type is explained outside the builtin function section.

Written by the indexing model from the issue text.