only allow chunk_size_gb for importable exports

Fixes: theforeman#1738

changelogs/fragments/1738-export_chunk_size.yml

@@ -0,0 +1,2 @@
+minor_changes:
+  - content_export_* - document that ``chunk_size_gb`` parameter is only applicable for ``importable`` exports (https://github.com/theforeman/foreman-ansible-modules/issues/1738)

plugins/doc_fragments/foreman.py

@@ -392,6 +392,7 @@ class ModuleDocFragment(object):
   chunk_size_gb:
     description:
       - Split the exported content into archives no greater than the specified size in gigabytes.
+      - Only applicable for C(format=importable). C(syncable) exports can't be split.
     required: false
     type: int
   format:

plugins/module_utils/foreman_helper.py

@@ -1585,6 +1585,9 @@ def run(self, **kwargs):
         if self.params.get('from_history_id') and incremental is not True:
             self.fail_json(msg='from_history_id is only valid for incremental exports')
 
+        if 'chunk_size_gb' in self.foreman_params and self.foreman_params['format'] == 'syncable':
+            self.fail_json(msg='chunk_size_gb is only valid for importable exports')
+
         self.auto_lookup_entities()
 
         payload = _flatten_entity(self.foreman_params, self.foreman_spec)