Don't overwrite unless existing entry is subheader
Don't overwrite unless existing entry is subheader

Should fix #3083

michaelclelland/documentation-generator,nickanderson/documentation-generator,stweil/documentation-generator,michaelclelland/documentation-generator,nickanderson/documentation-generator,cfengine/documentation-generator,cfengine/documentation-generator,stweil/documentation-generator,stweil/documentation-generator,michaelclelland/documentation-generator,cfengine/documentation-generator,stweil/documentation-generator,nickanderson/documentation-generator,cfengine/documentation-generator,cfengine/documentation-generator,michaelclelland/documentation-generator,stweil/documentation-generator,nickanderson/documentation-generator,michaelclelland/documentation-generator,nickanderson/documentation-generator