add: change advice config variables used by the add API

advice.addNothing config variable is used to control the visibility of
two advice messages in the add library. This config variable is
replaced by two new variables, whose names are more clear and relevant
to the two cases.

Also add the two new variables to the documentation.

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/advice.h b/advice.h
index 83287b0..0e6e58d 100644
--- a/advice.h
+++ b/advice.h
@@ -31,7 +31,8 @@ extern int advice_graft_file_deprecated;
 extern int advice_checkout_ambiguous_remote_branch_name;
 extern int advice_nested_tag;
 extern int advice_submodule_alternate_error_strategy_die;
-extern int advice_add_nothing;
+extern int advice_add_ignored_file;
+extern int advice_add_empty_pathspec;
 
 int git_default_advice_config(const char *var, const char *value);
 __attribute__((format (printf, 1, 2)))