Drop builtin policy "full" documentation

The full policy documentation was moved to Help/policy/*.rst by commit
f051814e (Convert builtin help to reStructuredText source files,
2013-10-15).  We no longer need the builtin string literals.

In commit 87cc62ca (Drop "full" documentation output types, 2013-09-13)
we dropped code using the LongDescription field of policy definitions.
We need to follow it up with a change similar to commit 399e9c46 (Drop
builtin property documentation, 2013-09-16) to remove the policy
documentation.  Do that now.  Keep the short description as it is used
in policy error and warning messages.
Revert "Handle genexes when evaluating INTERFACE_INCLUDE_DIRECTORIES errors."

This reverts commit 5945ee44244b0a09af3e35a1d7018f606f681ff5.
It will be rebased after policy documentation cleanup.
Merge topic 'doc-drop-policy-builtin' into next

5cdaef7 Drop builtin policy "full" documentation
Merge topic 'INTERFACE-error-with-linked-genex' into next

e315bab Revert "Handle genexes when evaluating INTERFACE_INCLUDE_DIRECTORIES errors."