Document why core signatures leave out methods defined only under -n/-p

[zonuexe]

Ruby defines the module functions Kernel?.sub, Kernel?.gsub, Kernel?.chop and Kernel?.chomp only when it runs with -n or -p (ruby.c). An ordinary script doesn't have them:

$ ruby -e 'p respond_to?(:gsub, true)'
false
$ echo x | ruby -ne 'p respond_to?(:gsub, true)'
true

core/kernel.rbs already leaves these methods out, but no document says so. If you compare the C source with RBS, they show up as missing signatures.

This PR adds a Q&A entry to docs/CONTRIBUTING.md. It explains that RBS doesn't declare them because a declaration would let type checkers accept calls that raise NoMethodError.