date_core.c: expand docs for Date shifting
* ext/date/date_core.c: [DOC] expand docs for Date shifting
* add examples for Date#>> and Date#<< that clarify some edge cases
* add examples for Date#next_year and Date#prev_year
* add cross references to Date#>> and Date#<<
[ruby-core:79584] [Bug #13225]
merge revision(s) 57737: [Backport #13225]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@58125 b2dd03c8-39d4-4d8f-98ff-823fe69b080e