One update for sure, another thing to be tested which may lead to an update,
on the page REST API

"JSON first instead of XML"

See inline comment from ahietala (https://goo.gl/tjvqpg)

"The example given for creating/updating nodes is incorrect"

at least that is what a user claims with a comment. See comment form Chris Jennings.
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Please try out. If required, update the appropriate section on the page.
In any case answer Chris.

Note that this (thing mentioned by Chris) could be related to MGNLREST-83 ("JSON output suddenly changed without changing API version number").
Please test the create/update example with a current 5.5.2 bundle which contains the problem which will be resolved with MGNLREST-83.
Then we have to test again, once MGNLREST-83 is resolved. This 2nd test can be done with a SNAPSHOT which integrates the REST fix.