Improve comments in cc/library.go and cc/linkable.go.

Test: N/A
Change-Id: I10cca7942bf86be8c933fd924f4bb7bdaff0cef7
diff --git a/cc/linkable.go b/cc/linkable.go
index 0609b28..ddf3950 100644
--- a/cc/linkable.go
+++ b/cc/linkable.go
@@ -6,6 +6,7 @@
 	"github.com/google/blueprint"
 )
 
+// LinkableInterface is an interface for a type of module that is linkable in a C++ library.
 type LinkableInterface interface {
 	Module() android.Module
 	CcLibrary() bool
@@ -51,23 +52,30 @@
 }
 
 var (
+	// Dependency tag for crtbegin, an object file responsible for initialization.
 	CrtBeginDepTag = dependencyTag{name: "crtbegin"}
-	CrtEndDepTag   = dependencyTag{name: "crtend"}
+	// Dependency tag for crtend, an object file responsible for program termination.
+	CrtEndDepTag = dependencyTag{name: "crtend"}
+	// Dependency tag for coverage library.
 	CoverageDepTag = dependencyTag{name: "coverage"}
 )
 
+// SharedDepTag returns the dependency tag for any C++ shared libraries.
 func SharedDepTag() blueprint.DependencyTag {
 	return libraryDependencyTag{Kind: sharedLibraryDependency}
 }
 
+// StaticDepTag returns the dependency tag for any C++ static libraries.
 func StaticDepTag() blueprint.DependencyTag {
 	return libraryDependencyTag{Kind: staticLibraryDependency}
 }
 
+// HeaderDepTag returns the dependency tag for any C++ "header-only" libraries.
 func HeaderDepTag() blueprint.DependencyTag {
 	return libraryDependencyTag{Kind: headerLibraryDependency}
 }
 
+// SharedLibraryInfo is a provider to propagate information about a shared C++ library.
 type SharedLibraryInfo struct {
 	SharedLibrary           android.Path
 	UnstrippedSharedLibrary android.Path
@@ -80,22 +88,30 @@
 
 var SharedLibraryInfoProvider = blueprint.NewProvider(SharedLibraryInfo{})
 
-type SharedLibraryImplementationStubsInfo struct {
-	SharedLibraryStubsInfos []SharedLibraryStubsInfo
-
-	IsLLNDK bool
-}
-
-var SharedLibraryImplementationStubsInfoProvider = blueprint.NewProvider(SharedLibraryImplementationStubsInfo{})
-
-type SharedLibraryStubsInfo struct {
+// SharedStubLibrary is a struct containing information about a stub shared library.
+// Stub libraries are used for cross-APEX dependencies; when a library is to depend on a shared
+// library in another APEX, it must depend on the stub version of that library.
+type SharedStubLibrary struct {
+	// The version of the stub (corresponding to the stable version of the shared library being
+	// stubbed).
 	Version           string
 	SharedLibraryInfo SharedLibraryInfo
 	FlagExporterInfo  FlagExporterInfo
 }
 
-var SharedLibraryStubsInfoProvider = blueprint.NewProvider(SharedLibraryStubsInfo{})
+// SharedLibraryStubsInfo is a provider to propagate information about all shared library stubs
+// which are dependencies of a library.
+// Stub libraries are used for cross-APEX dependencies; when a library is to depend on a shared
+// library in another APEX, it must depend on the stub version of that library.
+type SharedLibraryStubsInfo struct {
+	SharedStubLibraries []SharedStubLibrary
 
+	IsLLNDK bool
+}
+
+var SharedLibraryStubsProvider = blueprint.NewProvider(SharedLibraryStubsInfo{})
+
+// StaticLibraryInfo is a provider to propagate information about a static C++ library.
 type StaticLibraryInfo struct {
 	StaticLibrary android.Path
 	Objects       Objects
@@ -109,10 +125,12 @@
 
 var StaticLibraryInfoProvider = blueprint.NewProvider(StaticLibraryInfo{})
 
+// FlagExporterInfo is a provider to propagate transitive library information
+// pertaining to exported include paths and flags.
 type FlagExporterInfo struct {
-	IncludeDirs       android.Paths
-	SystemIncludeDirs android.Paths
-	Flags             []string
+	IncludeDirs       android.Paths // Include directories to be included with -I
+	SystemIncludeDirs android.Paths // System include directories to be included with -isystem
+	Flags             []string      // Exported raw flags.
 	Deps              android.Paths
 	GeneratedHeaders  android.Paths
 }