cmake: fix missing addition of policies to documentation index
Summary:
Since cmake 3.0 cmake also supports --help-policy-list, matching the
general --help* pattern. The KDevelop cmake documentation plugin
already has an entry for "Policies", just it is not filled with any
content.
This patch extends the content indexing by also including the policy.
For cmake < 3.0 the process CMakeCommandsContents::processOutput will
catch the failing call by the exit code, and things are as before.
Reviewers: KDevelop, apol
Reviewed By: KDevelop, apol
Subscribers: kdevelop-devel
Tags: KDevelop
Differential Revision: https://phabricator.kde.org/D15882
(cherry picked from commit 2f0f3da749217bb0e9c2b5de9e4b662a3e549129)