doc: improve conciseness
doc: improve conciseness

The object_to_key_iter() example is now formatted like
json_object_foreach() and json_object_foreach(). The edited descriptions were
redundant, and the removed headers didn't add useful information.

akheron/jansson,akheron/jansson,akheron/jansson,npmccallum/jansson,markalanj/jansson,wirebirdlabs/featherweight-jansson,npmccallum/jansson,Mephistophiles/jansson,bstarynk/jansson,Mephistophiles/jansson,markalanj/jansson,bstarynk/jansson,AmesianX/jansson,bstarynk/jansson,wirebirdlabs/featherweight-jansson,markalanj/jansson,AmesianX/jansson