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/llndk_library.go b/cc/llndk_library.go
index 5a36b7f..3e25481 100644
--- a/cc/llndk_library.go
+++ b/cc/llndk_library.go
@@ -189,6 +189,14 @@
 	return module
 }
 
+// llndk_library creates a stub llndk shared library based on the provided
+// version file. Example:
+//
+//    llndk_library {
+//        name: "libfoo",
+//        symbol_file: "libfoo.map.txt",
+//        export_include_dirs: ["include_vndk"],
+//    }
 func LlndkLibraryFactory() android.Module {
 	module := NewLLndkStubLibrary()
 	android.InitAndroidArchModule(module, android.DeviceSupported, android.MultilibBoth)
@@ -203,6 +211,8 @@
 	return name + llndkHeadersSuffix
 }
 
+// llndk_headers contains a set of c/c++ llndk headers files which are imported
+// by other soongs cc modules.
 func llndkHeadersFactory() android.Module {
 	module, library := NewLibrary(android.DeviceSupported)
 	library.HeaderOnly()