Add a precision that number_sections default to TRUE and this will change outputs

Author: cderv

NEWS.md

@@ -2,7 +2,7 @@
 
 ## NEW FEATURES
 
-- Add the `number_sections` argument to `markdown_document2()` and its family. This allows to have now figure references numbered by chapters in these formats, like `word_document2()` or `odt_document2()` for example (thanks, @atusy, #756).
+- Add the `number_sections` argument to `markdown_document2()` and its family. This allows to have now figure references numbered by chapters in these formats, like `word_document2()` or `odt_document2()` for example. This argument default to `TRUE` like `html_document2()` and  `pdf_document2()`. Set it to `number_sections = FALSE` to get the same output as previous version without numbered chapters (thanks, @atusy, #756). 
 
 ## BUG FIXES