Soong: Add synopsis to several modules under the cc package.

Added synopsis to the following modules under cc package:
    * llndk_headers
    * llndk_library
    * vendor_public_library
    * toolchain_library

Bug: b/128337482
Test: Generated the documentation and verified that the synopsis was
added to each of the module.

Change-Id: I0f6e3b01b478d46afeacbe6fc8e05f96d6217c7d
diff --git a/cc/vendor_public_library.go b/cc/vendor_public_library.go
index da41cbc..2072ad9 100644
--- a/cc/vendor_public_library.go
+++ b/cc/vendor_public_library.go
@@ -122,6 +122,15 @@
 	return stub.libraryDecorator.link(ctx, flags, deps, objs)
 }
 
+// vendor_public_library creates a stub shared library for a vendor public
+// library. This stub library is a build-time only artifact that provides
+// symbols that are exposed from a vendor public library. Example:
+//
+//    vendor_public_library {
+//        name: "libfoo",
+//        symbol_file: "libfoo.map.txt",
+//        export_public_headers: ["libfoo_headers"],
+//    }
 func vendorPublicLibraryFactory() android.Module {
 	module, library := NewLibrary(android.DeviceSupported)
 	library.BuildOnlyShared()