bug#34391: 26.1; [[:cntrl:]] does not match DEL contrary to documentation

[Mattias Engdegård]

Unlike every other regexp engines and POSIX regexps, and contrary to the 
documentation, [[:cntrl:]] does not match DEL (\177) in Emacs. (It does not 
match the C1 controls in U+0080..009f either, but at least there is no such 
claim.)

Assuming that it is not worth breaking existing code by changing the behaviour, 
let us at least fix the manual which says:

‘[:cntrl:]’
     This matches any ASCII control character.

which is inaccurate. The error also made it into the doc string of `rx'.