Auto merge of #3955 - RReverser:update-override-docs, r=alexcrichton
Auto merge of #3955 - RReverser:update-override-docs, r=alexcrichton

Update documentation on overrides

 - `target.linker` does not seem to be supported anymore, only `target.$triple.linker` works, so merged these sections.
 - Added note about supported `target.$triple.ar`

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