Most of the manifesto is background and detailed definitions -- if you're confused or want details, read the manifesto!
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170213/032155.html
Note also that manifestos aren't complete proposals -- syntax and details may change!
One piece of background: inout is kinda complicated because it can be used on computed properties -- foo(&val.x) might be sugar for