Fix bug #10129: add positional information to "C-u C-x =".

Fix bug #10129: add positional information to "C-u C-x =".

 lisp/descr-text.el (describe-char): Show information about POS, in
 addition to information about the character at POS.  Improve and
 update the doc string.  Change "code point" to "code point in
 charset", to avoid confusion with the character's Unicode code
 point shown above that.