Fix API doc's content and style for enum. FT mapping iterator

Fix API doc's content and style for enum. FT mapping iterator

Also, remove bt_ctf_field_type_enumeration_mapping_iterator_get_name(),
since you can achieve the same with
bt_ctf_field_type_enumeration_mapping_iterator_get_signed() or
bt_ctf_field_type_enumeration_mapping_iterator_get_unsigned() with the
two last parameters set to NULL.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>