Adds file doc/case_mapping.rdoc, which describes case mapping and provides a link target that methods doc can link to.

Revises:

    String#capitalize
    String#capitalize!
    String#casecmp
    String#casecmp?
    String#downcase
    String#downcase!
    String#swapcase
    String#swapcase!
    String#upcase
    String#upcase!
    Symbol#capitalize
    Symbol#casecmp
    Symbol#casecmp?
    Symbol#downcase
    Symbol#swapcase
    Symbol#upcase