[boost] 1.59 release notes don't mention new property_tree::json_parser

The commit logs say: TL;DR: The new parser fixes long-standing bugs and has full Unicode support, but removes non-standard extensions of the old parser, which could break code: - String concatenation: the old parser concatenated adjacent string literals like C does. - Comments: the old parser supported C and C++-style comments. JSON doesn't allow comments. Removing support for comments did break something. Unfortunately there was no sign of this in the release notes, so I had to debug it the hard way (in code that swallowed the exception from the parser - gah!) and only found the commit message once I already knew what the problem was. </rant>