Hash: Document quoted-string-followed-by-colon syntax for Symbol keys in Hash literals

[das-g]

In Ruby >= 2.2, the following Hash literals are equivalent:

1. { :my_key => 5 }
   
   (Symbol literal used as key in the generic hash literal notation that'd work for non-Symbol keys, too.)
2. { my_key: 5 }
   
   (Symbol-key-specific "JavaScript-like" new notation introduced in Ruby 1.9)
3. { :"my_key" => 5 }
   
   (Same as (1.), just with a different notation for the Symbol literal. Allows string interpolation inside the symbol name.)
4. { :'my_key' => 5 }
   
   (Same as (1.), just with a different notation for the Symbol literal. No string interpolation.)
5. { "my_key": 5 }
   
   (Introduced by feature request #4276 in Ruby 2.2. Allows string interpolation inside the symbol name.)
6. { 'my_key': 5 }
   
   (Probably also introduced by feature request Move rb_int_coerce ruby/ruby#4276. No string interpolation.)

While (1.) & (2.) are documented on http://ruby-doc.org/core-2.2.2/Hash.html and (3.) & (4.) follow from (1.) when plugging in the alternate Symbol literal notations, (5.) & (6.) seem to be undocumented yet.