Doc: unstable book - profile.md: improve wording
Doc: unstable book - profile.md: improve wording

- mention `--target` flag excludes RUSTFLAGS passing to build scripts and proc macros

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust