Fxfs is currently an experimental filesystem that might end up going nowhere. It is currently being used to explore ideas and may end up being the basis of a Minfs replacement. If and when that happens, RFCs will cover the design.
Fxfs has rustdoc style documentation which can be generated and opened with command like
fx rustdoc //src/storage/fxfs:lib --open
Note: The last line prints the documentation location, which depends on target.