docs(core): tighten Ruby group rationale to reflect measured shape

The initial comment was speculative ("individual files sometimes leak
elsewhere"). Measurement across 10 major Ruby repos showed the picture
is much stronger and closer to C++ than to Rust:

  Weighted total reduction across the sample: 51% (−11.80M tokens on
  92 MB of Ruby source), the highest of any language group so far.
  spec/ dir rule alone drives 46 pp; file globs add the remaining 5 pp
  by catching *_spec.rb in gem lib/ trees, Minitest files leaking
  outside test/ in Rails engines, and helper bootstraps referenced
  via require_relative.

  Hero projects:
    rubocop/rubocop        67% (−1.75M tok)  highest percent
    discourse/discourse    60% (−5.53M tok)  highest absolute
    ruby/ruby              59% (−3.66M tok)
    rspec/rspec-rails      61% (−0.07M tok)
    fastlane/fastlane      42% (−0.73M tok)

Update the block comment so a future reader lands on the right mental
model — Ruby is a big win precisely because the ecosystem clusters
tests but under a directory (`spec/`) that the starter had previously
missed entirely.