Redmine#4834: document tags
Merge pull request #353 from tzz/doc/redmine4834-tags

Redmine#4834: document tags
Fix formatting of XML string in example.

Probably the result of improper conversion or copy/paste from HTML.
(cherry picked from commits c15715014cbae7f9ff05e49d774372fdd45277d1
and b3259d18ca3453442d9b505d787c56d987c7c8ee)