Improve documentation on type declaration

[johannes-lindgren]

Declaring the type (and thus type checking your validation functions) has a great benefit when

• you're writing libraries and don't want to leak the validation library to the library consumer
• you're adopting validation to an existing code base and don't want to throw away your exiting types
• This is work-in-progress: https://pure-parse.vercel.app/guide/declaring-vs-inferring.html#decoupling
• This might no longer work: https://pure-parse.vercel.app/guide/declaring-vs-inferring.html#recursive-types