Added comments about utility folder role in library discovery

Author: cmaglie

legacy/builder/container_find_includes.go

@@ -312,6 +312,10 @@ func findIncludesUntilDone(ctx *types.Context, cache *includeCache, sourceFile *
 	for {
 		cache.ExpectFile(sourcePath)
 
+		// Libraries may require the "utility" directory to be added to the include
+		// search path, but only for the source code of the library, so we temporary
+		// copy the current search path list and add the library' utility directory
+		// if needed.
 		includes := ctx.IncludeFolders
 		if library := sourceFile.Library; library != nil && library.UtilityDir != nil {
 			includes = append(includes, library.UtilityDir)