Change Password Hint

Just add a filter, where you can change the text, like this:

add_filter( 'password_hint', function( $hint )
{
  return __( 'MY OWN PASSWORD HINT' );
} );

This can be added in functions.php in your theme.

A bit of an explanation
there in core you can see:

return apply_filters( 'password_hint', $hint );

that is where the function will be applied.