* Document Range#to_set

* Update Thread#raise and Fiber#raise signatures and docs

* Add reference to String#strip to character_selectors.rdoc

* Update *nil docs when calling methods

* Enhance Array#find and #rfind docs

* Add a notice to Kernel#raise about cause:

Otherwise Array#detect is Enumerable#detect while Array#find uses a
different more performant implementation.

[Feature #21678]

Implement Array#rfind, which is the same as find except from the
other side of the Array. Also implemented Array#find (as opposed to
the generic one on Enumerable because it is significantly faster
and to keep the implementations together.

[Feature #21678]

When all elements are strings, we never have to recalculate the length
of the array because there are no conversion methods that are called, so
the length will never change. This speeds up the fast path by ~10%.

```ruby
a = ["1"*10, "2"*10, "3"*10, "4"*10, "5"*10] * 10
10_000_000.times do
  a.join
end
```

```
hyperfine --warmup 1 'ruby ../ruby2/test.rb' './exe/ruby ../ruby2/test.rb'
Benchmark 1: ruby ../ruby2/test.rb
  Time (mean ± σ):      3.779 s ±  0.053 s    [User: 3.754 s, System: 0.017 s]
  Range (min … max):    3.715 s …  3.874 s    10 runs

Benchmark 2: ./exe/ruby ../ruby2/test.rb
  Time (mean ± σ):      3.411 s ±  0.038 s    [User: 3.387 s, System: 0.017 s]
  Range (min … max):    3.360 s …  3.472 s    10 runs

Summary
  ./exe/ruby ../ruby2/test.rb ran
    1.11 ± 0.02 times faster than ruby ../ruby2/test.rb
```

Re-organize page docs

We need to allocate at least sizeof(struct RArray) when the array is
embedded on garbage collectors that support slot sizes less than 40 bytes.

to adopt strict shareable rule.

* (basically) shareable objects only refer shareable objects
* (exception) shareable objects can refere unshareable objects
  but should not leak reference to unshareable objects to Ruby world

Duplicate the was found in the documentation