(PUP-9603) Update outdated specification of heredoc syntax tag

[hlindberg]

This updates the specification of the heredoc syntax tag: @(MARKER:<syntax>).

The text described the now deprecated experimental "binder" and how it is used to extend the set of available syntax checkers. The binder was dropped in Puppet 5 in favor of a much simpler mechanism. The replacement mechanism (based on Puppet context) does not have the ability to handle extensions (other than by modifying the source to push additional things onto the Puppet context and this is not available to users via configuration).

This commit updates the specification text to what is currently supported and it enumerates the checkers available in puppet core.

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