where is the documentation for add_action() parameters?

those parameters aren’t unique to the_posts, they’re parameters for the add_action() function

add_action( $tag, $function_to_add, $priority, $accepted_args ); 

the 10 is just the default priority and 1 is the default number of accepted arguments. since they’re both default, you don’t really need either.

http://codex.wordpress.org/Function_Reference/add_action

tech