Update toolchains documentation for new optional toolchains feature.
Update toolchains documentation for new optional toolchains feature.

Part of https://github.com/bazelbuild/bazel/issues/14726

RELNOTES: Toolchain types may now be optional, in addition to mandatory. See https://bazel.build/docs/toolchains#optional-toolchains for further details.
PiperOrigin-RevId: 446279106

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