doc: Replace *_thread_resource_pool_assign with k_thread_heap_assign

Replace *_thread_resource_pool_assign() in the reference with the new
k_thread_heap_assign() since both k_thread_resource_pool_assign() and
z_thread_resource_pool_assign() has been removed prio to v2.5 (by the
commit c770cab1 and 3c2c1d85 respectively) along with the
k_mem_pool API removal.

For the resource pool inheritance test, the variables with "res_pool"
string has been replaced by "heap_mem" to align with the documentation
fix.  No functionality has been changed.

Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>