message: 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
diff: @@ -60,16 +60,12 @@ vcs = "none"
  # literal string "$triple", and it will apply whenever that target triple is
  # being compiled to. 'cfg(...)' refers to the Rust-like `#[cfg]` syntax for
  # conditional compilation.
  -[target]
  -# For Cargo builds which do not mention --target, this is the linker
  -# which is passed to rustc (via `-C linker=`). By default this flag is not
  -# passed to the compiler.
  -linker = ".."
  -
  [target.$triple]
  -# Similar to the above linker configuration, but this only applies to
  -# when the `$triple` is being compiled for.
  +# This is the linker which is passed to rustc (via `-C linker=`) when the `$triple`
  +# is being compiled for. By default this flag is not passed to the compiler.
  linker = ".."
  +# Same but for the library archiver which is passed to rustc via `-C ar=`.
  +ar = ".."
  # custom flags to pass to all compiler invocations that target $triple
  # this value overrides build.rustflags when both are present
  rustflags = ["..", ".."]
  