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/toolchain_library.go b/cc/toolchain_library.go
index ae08b1c..6494a26 100644
--- a/cc/toolchain_library.go
+++ b/cc/toolchain_library.go
@@ -48,6 +48,9 @@
 	return append(props, &library.Properties)
 }
 
+// toolchain_library is used internally by the build tool to link the specified
+// static library in src property to the device libraries that are shipped with
+// gcc.
 func ToolchainLibraryFactory() android.Module {
 	module, library := NewLibrary(android.HostAndDeviceSupported)
 	library.BuildOnlyStatic()