Unify `cattr_*` interface: allow to pass a block to `cattr_reader`.

Example:

    class A
      cattr_reader(:defr) { 'default_reader_value' }
    end
    A.defr # => 'default_reader_value'
Remove broken Commmand Line guide link

[ci skip]
added one test case and example for ActiveRecord::Base.to_param method
Merge pull request #12899 from nateberkopec/remove_broken_internal_guide_link

Remove broken Commmand Line guide link
Merge pull request #12900 from kuldeepaggarwal/f-to-param

added one test case and example for ActiveRecord::Base.to_param method
explain how `to_param` wil truncate long values. [ci skip] refs #12900.
Merge pull request #10848 from 907th/add_block_to_cattr_reader

Fix #10847: allow to pass a block to `cattr_reader`.