Various corrections and enhancements to the documentation.

Change-Id: I03519bfccf4367b36d36006f1450d5fbcbbf8621
diff --git a/include/ceres/autodiff_local_parameterization.h b/include/ceres/autodiff_local_parameterization.h
index 1099061..0aae6c7 100644
--- a/include/ceres/autodiff_local_parameterization.h
+++ b/include/ceres/autodiff_local_parameterization.h
@@ -58,7 +58,7 @@
 //
 // For example, Quaternions have a three dimensional local
 // parameterization. It's plus operation can be implemented as (taken
-// from interncal/ceres/auto_diff_local_parameterization_test.cc)
+// from internal/ceres/auto_diff_local_parameterization_test.cc)
 //
 //   struct QuaternionPlus {
 //     template<typename T>