Add warning to the jit mode section of the usage docs
Add warning to the jit mode section of the usage docs

Closes #1192
graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust