string.c: [DOC] fix doc formatting for String#==, #===
	math.c: more accuracy

	* math.c (math_log10): calculate log10(2) for more accuracy.
	  [ruby-core:79907] [Bug #13279]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57928 b2dd03c8-39d4-4d8f-98ff-823fe69b080e