Add description of new value AUTO_DELETE for table property pna_idle_timeout

[jfingerh]

Also clarify some wording in the description of add-on-miss, and remove the old signature in the document, in favor of using Madoko INCLUDE mechanism to include the corresponding section of the pna.p4 include file, so that the document and include file will remain in sync with each other.

[thomascalvert-xlnx]

LGTM.

(It did leave me wondering whether the pna_idle_timeout and idle_timeout_with_auto_delete properties could be unified into a single one... (Maybe make AUTO_DELETE an additional member of the PNA_IdleTimeout_t enum?) But that's not the topic of this commit. :) )

[jfingerh]

@thomascalvert-xlnx I like the idea of making AUTO_DELETE a new member of the enum PNA_IdleTimeout_t. I suppose some targets might even be able to implement a combination of notifying the control plane AND auto-deleting the entry when an entry expires, but no need to add that unless there is a request to do so.

I will go ahead and change this PR with this idea, and see what others reviewers think.

[thantry]

LGTM