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