doc: memory_domain: Update memory domain API's documentation

Remove thread from memory domain API (k_mem_domain_remove_thread())has
only one argument which is thread ID as per the implementation whereas
documentation says there has to be two arguments, memory domain and
thread ID.Memory domain argument is not required as a thread belongs
to single memory domain at any point in time. Also memory domain
initialisation function (k_mem_domain_init()) should accept only 3
arguments i.e, memory domain name, number of parts and array of
pointers to the memory domain, instead of 4.

Signed-off-by: Spoorthi K <spoorthi.k@intel.com>