xmldocs: Add support for an <example> tag in the Asterisk XML Documentation

This patch adds support for an <example /> tag in the XML documentation schema.

For CLI help, this doesn't change the formatting too much:
 - Preceeding white space is removed
 - Unlike with para elements, new lines are preserved

However, having an <example /> tag in the XML schema allows for the wiki
documentation generation script to surround the documentation with {code} or
{noformat} tags, generating much better content for the wiki - and allowing us
to put dialplan examples (and other code snippets, if desired) into the
documentation for an application/function/AMI command/etc.

Review: https://reviewboard.asterisk.org/r/3807/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@419822 65c4cc65-6c06-0410-ace0-fbb531ad65f3