Help: Drop FeatureSummary example in cmake-developer.7

The use of FeatureSummary inside a find module is not a convention
yet used by upstream CMake modules.  Drop the example from the
documentation about how to write find modules.  If in the future
we add use of FeatureSummary to many of the upstream find modules
then this example can be restored as part of establishing the
convention.
Merge branch 'backport-doc-find-module-no-summary' into release
Help: Extend policy CMP0054 documentation with an example
Merge branch 'doc-CMP0054-example' into release