This commit does the following:
+ some rewordings, simplifications and typo fixes in "Users and Basic
Account Management" section (discussed with wblock in ML [1]);
+ adds xml:ids to three examples and makes use of them;
+ nothing around "becoming superuser" -- that subchapter was rearranged
in r42873;
+ deletes suddenly revived handbook/users directory (the first attempt
was in r42953).