RDoc of Random::Formatter

[aycabta]

https://bugs.ruby-lang.org/issues/14576 is reported by @znz:

https://docs.ruby-lang.org/en/trunk/Random/Formatter.html has following methods:

#alphanumeric
#base64
#choose
#gen_random
#hex
#rand
#random_bytes
#random_number
#urlsafe_base64
#uuid

But Random::Formatter.instance_methods are only two methods.

>> Random::Formatter.instance_methods
=> [:random_number, :rand]

[aycabta]

@nobu commented at https://bugs.ruby-lang.org/issues/14576:

The other methods are defined in securerandom.rb.