Add implementation notes for toolchain resolution.
Add implementation notes for toolchain resolution.

This is deliberately not linked from other docs yet, trying to figure out the best place for it.

PiperOrigin-RevId: 425910575

katre/bazel,katre/bazel,cushon/bazel,katre/bazel,bazelbuild/bazel,ButterflyNetwork/bazel,katre/bazel,katre/bazel,bazelbuild/bazel,bazelbuild/bazel,cushon/bazel,cushon/bazel,cushon/bazel,ButterflyNetwork/bazel,cushon/bazel,ButterflyNetwork/bazel,ButterflyNetwork/bazel,ButterflyNetwork/bazel,ButterflyNetwork/bazel,katre/bazel,bazelbuild/bazel,bazelbuild/bazel,bazelbuild/bazel,cushon/bazel