Document the descriptor set output of proto_library.
--
PiperOrigin-RevId: 149659956
MOS_MIGRATED_REVID=149659956
diff --git a/src/main/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibraryRule.java b/src/main/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibraryRule.java
index cbb09fd..c0a6eb0 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibraryRule.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/proto/BazelProtoLibraryRule.java
@@ -94,4 +94,14 @@
in the <code>deps</code> clause of supported rules, such as <code>java_proto_library</code>.
</p>
+<p>When compiled on the command-line, a <code>proto_library</code> creates a file named
+<code>foo-descriptor-set.proto.bin</code>, which is the descriptor set for the
+messages the rule srcs. The file is a serialized <code>FileDescriptorSet</code>, which is described
+in https://developers.google.com/protocol-buffers/docs/techniques#self-description.</p>
+
+<p>It only contains information about the <code>.proto</code> files directly mentioned by a
+<code>proto_library</code> rule; the collection of transitive descriptor sets is available through
+the <code>proto.transitivedescriptorsets</code> Skylark provider.
+See documentation in <code>ProtoSourcesProvider.java</code>.</p>
+
<!-- #END_BLAZE_RULE -->*/
