Document the descriptor set output of proto_library.
MOS_MIGRATED_REVID=149659956
@@ -94,4 +94,14 @@ public final class BazelProtoLibraryRule implements RuleDefinition {
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 -->*/
