Improve StructType documentation

[sitio-couto]

LGTM with one minor addition: add some doc bits to tablegen about how structs can be seen in the wild. E.g. it might not be obvious why anonymous structs must be complete. You can pick adding to this PR, or creating a new one.

Originally posted by @bcardosolopes in https://github.com/llvm/clangir/pull/294#pullrequestreview-1709200291