DnfPackage: add string lifetime notices to descriptions
DnfPackage: add string lifetime notices to descriptions

Unlike other getters, `dnf_package_get_nevra` uses `pool_solvable2str`,
which actually builds the NEVRA on-the-fly and stores it in the pool's
temporary buffer. The actual pointer returned can become invalid simply
by subsequent calls to `pool_solvable2str` until a `realloc` is
triggered. This can lead to undefined behaviour in clients that assume
the string has at least the same lifetime as the `DnfPackage` object.

Let's add a notice to the API description to make sure clients are aware
of this limitation.

edynox/libdnf,Conan-Kudo/libhif,Conan-Kudo/libhif,rpm-software-management/libhif,edynox/libdnf,Conan-Kudo/libhif,rpm-software-management/libhif,rpm-software-management/libdnf,rpm-software-management/libhif,rpm-software-management/libdnf,hughsie/libhif,rpm-software-management/libdnf,edynox/libdnf,edynox/libdnf,hughsie/libhif,rpm-software-management/libhif,hughsie/libhif,rpm-software-management/libdnf,Conan-Kudo/libhif,hughsie/libhif