WASI documentation includes:

• Overview: provides an introduction to WASI and its history
• Goals: a succinct list of WASI's design goals
• Design Principles: discusses details on the principles of capability-based security, scope, POSIX/Web, etc.
• WITX: an introduction to the WITX specification language
• Process: describes how to move a WASI proposal in to the specification
• Proposals: lists the current WASI proposals by phase

Additionally, here are some links that may be helpful in understanding WASI:

• The blog post introducing WASI: Standardizing WASI: A system interface to run WebAssembly outside the web
• The wasi.dev web site includes links to more information about WASI, including how to get started using it
• This repository's issue tracker