hash.c: exception examples

	* hash.c (rb_hash_dig): [DOC] add examples of exceptions.
	Fix doc for Hash#dig and Struct#dig

	* hash.c (rb_hash_dig): [DOC] correct argument name
	  in method description; fix formatting in examples.
	* struct.c (rb_struct_dig): ditto.

	[ruby-core:79221] [Bug #13148]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@58153 b2dd03c8-39d4-4d8f-98ff-823fe69b080e