security: Make addKey/Cert behavior of PibImpl consistent

Change-Id: I30c6dd0007e9095ee07f7a8eda3b20db4c34c513
Refs: #3351
diff --git a/src/security/pib/pib.hpp b/src/security/pib/pib.hpp
index 3d73098..06f100d 100644
--- a/src/security/pib/pib.hpp
+++ b/src/security/pib/pib.hpp
@@ -128,7 +128,6 @@
   getDefaultIdentity() const;
 
 NDN_CXX_PUBLIC_WITH_TESTS_ELSE_PRIVATE: // write operations should be private
-
   /*
    * @brief Create an identity with name @p identityName and return a reference to it.
    *
@@ -143,6 +142,8 @@
   /*
    * @brief Remove an identity with name @p identityName.
    *
+   * If the default identity is being removed, no default identity will be selected.
+   *
    * @param identityName The name for the identity to be deleted
    */
   void