merge revision(s) 43669,43672,43678: [Backport #8958]

	* ext/bigdecimal/lib/bigdecimal/util.rb: [DOC] Document the required
	  +precision+ argument for Rational#to_d [Bug #8958]

	* ext/bigdecimal/lib/bigdecimal/util.rb: [DOC] +precision+ is required

	* ext/bigdecimal/lib/bigdecimal/util.rb: [DOC] remove example of
	  Rational#to_d without argument. [Bug #8958]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@44314 b2dd03c8-39d4-4d8f-98ff-823fe69b080e