Currently, the doc says that this methods returns `nil` if there is
no cpu_quota, but actually not.
https://github.com/ruby-concurrency/concurrent-ruby/blob/6f7c91ab5a4e99d850f69c958572a536426277e8/lib/concurrent-ruby/concurrent/utility/processor_counter.rb#L30

This PR fixes the doc to match the behavior.

Co-authored-by: Benoit Daloze <eregontp@gmail.com>

…cpu.cfs_quota_us` is -1

I tried to use `Concurrent.available_processor_count` in `parallel`
gem, but we got some feedback `Concurrent.available_processor_count`
returned a negative value.
grosser/parallel#348 (comment)
grosser/parallel#349 (comment)

According to the https://docs.kernel.org/scheduler/sched-bwc.html#management,
The default value of `cpu.cfs_quota_us` is -1. In that case,
cgroup does not adhere to any CPU time restrictions.

This PR adds the case of `cpu.cfs_quota_us` is -1 to `#cpu_quota`
to return processor count from `Concurrent.available_processor_count`
in that case.

* Like:
concurrent-ruby/vendor/bundle/ruby/3.2.0/gems/concurrent-ruby-1.3.4/lib/concurrent-ruby/concurrent/version.rb:2: warning: already initialized constant Concurrent::VERSION
concurrent-ruby/lib/concurrent-ruby/concurrent/version.rb:2: warning: previous definition of VERSION was here

…d warnings