doc: Nokogiri::XML::Node.traverse is recursively called on children first (#3480)

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**What problem is this PR intended to solve?**

The documentation of
[traverse](https://nokogiri.org/rdoc/Nokogiri/XML/Node.html#method-i-traverse)
says "Yields self and all children to block recursively". I thought that
means *first yield self, then recurse on children*. But actually it's
the other way around: *first recurse on children, then yield self*. This
PR changes the RDoc comment to clarify that.

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**Have you included adequate test coverage?**

not necessary for this PR

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**Does this change affect the behavior of either the C or the Java
implementations?**

no

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Author: flavorjones

lib/nokogiri/xml/node.rb

@@ -1339,7 +1339,7 @@ def ancestors(selector = nil)
       end
 
       ####
-      # Yields self and all children to +block+ recursively.
+      # Yields all children to +block+ recursively, then yields self.
       def traverse(&block)
         children.each { |j| j.traverse(&block) }
         yield(self)