Update docs

* bazelbuild/bazel#3867 is fixed, so
  instructions for vendored proto files need to be updated.
* proto_library is now provided by rules_proto, not built-in.

proto/core.rst

@@ -13,7 +13,7 @@ Go Protocol buffers
 .. _Bourne shell tokenization: https://docs.bazel.build/versions/master/be/common-definitions.html#sh-tokenization
 .. _gogoprotobuf: https://github.com/gogo/protobuf
 .. _compiler.bzl: compiler.bzl
-.. _bazelbuild/bazel#3867: https://github.com/bazelbuild/bazel/issues/3867
+.. _rules_proto: https://github.com/bazelbuild/rules_proto
 
 .. role:: param(kbd)
 .. role:: type(emphasis)
@@ -33,7 +33,7 @@ Overview
 Protocol buffers are built with the three rules below. ``go_proto_library`` and
 ``go_proto_compiler`` may be loaded from ``@io_bazel_rules_go//proto:def.bzl``.
 
-* `proto_library`_: This is a Bazel built-in rule. It lists a set of .proto
+* `proto_library`_: This is provided by rules_proto_. It lists a set of .proto
   files in its ``srcs`` attribute and lists other ``proto_library`` dependencies
   in its ``deps`` attribute. ``proto_library`` rules may be referenced by
   language-specific code generation rules like ``java_proto_library`` and
@@ -140,15 +140,13 @@ have protos, you'll also need to add
 A note on vendored .proto files
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Bazel can only handle imports in .proto files that are relative to a repository
+By default, Bazel assumes imports in .proto files are relative to a repository
 root directory. This means, for example, if you import ``"foo/bar/baz.proto"``,
 that file must be in the directory ``foo/bar``, not
 ``vendor/example.com/repo/foo/bar``.
 
-If you have proto files that don't conform to this convention, follow the
-instructions for using pre-generated .pb.go files above.
-
-The Bazel tracking issue for supporting this is `bazelbuild/bazel#3867`_.
+To deal with this, use the `strip_import_prefix` option in the proto_library_
+for the vendored file.
 
 API
 ---
@@ -365,7 +363,7 @@ go_proto_compiler
 ``go_proto_compiler`` describes a plugin for protoc, the proto compiler.
 Different plugins will generate different Go code from the same protos.
 Compilers may be chosen through the ``compilers`` attribute of
-``go_proto_library``. 
+``go_proto_library``.
 
 Several instances of this rule are listed in `Predefined plugins`_. You will
 only need to use this rule directly if you need a plugin which is not there.
@@ -456,7 +454,7 @@ use protoc), you can write a new rule that produces this.
 ``GoProtoCompiler`` is loaded from ``@io_bazel_rules_go//proto:def.bzl``.
 
 ``GoProtoCompiler`` has the fields described below. Additional fields may be
-added to pass information to the ``compile`` function. This interface is 
+added to pass information to the ``compile`` function. This interface is
 *not final* and may change in the future.
 
 +-----------------------------+-------------------------------------------------+