Auto merge of #4355 - pornel:docs, r=alexcrichton
Auto merge of #4355 - pornel:docs, r=alexcrichton

Expand profile settings descriptions

Flags were described in the format `foobar = true # controls foobar, passes '--foobar' flag`, but this doesn't actually say what `foobar` does. I've expanded the descriptions a little to provide more context.

vojtechkral/cargo,nipunn1313/cargo,rust-lang/cargo,vojtechkral/cargo,tsurai/cargo,rust-lang/cargo,nipunn1313/cargo,achanda/cargo,eminence/cargo,rustyhorde/cargo,eminence/cargo,rustyhorde/cargo,rust-lang/cargo,tshepang/cargo,vojtechkral/cargo,nipunn1313/cargo,tshepang/cargo,achanda/cargo,tsurai/cargo,tsurai/cargo,tsurai/cargo,vojtechkral/cargo,achanda/cargo,achanda/cargo,tsurai/cargo,tshepang/cargo,rust-lang/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,tshepang/cargo,eminence/cargo,rustyhorde/cargo