When text-quoting-style was introduced, Stefan Monnier (then the 
maintainer) did not see the need the the variable at all, and wrote "At 
the very least it should not be a Custom var." See:


https://lists.gnu.org/archive/html/emacs-devel/2015-08/msg01020.html


Eventually I vaguely recall that Stefan was persuaded to put up with the 
variable. Still, I think he's right that it's not suitable for a user 
option. I installed the attached patch to try to document this better. 
Since it's intended for experts, the variable is documented in the 
reference manual, not the user manual.