hash: fix doc about thread/process safety

hash: fix doc about thread/process safety

rte_hash_hash is multi-thread safe but not multi-process safe
because of the use of function pointers. Previous document
and comment says the other way around. This commit fixes
the issue.

Fixes: fc1f2750a3ec ("doc: programmers guide")
Fixes: 48a399119619 ("hash: replace with cuckoo hash implementation")
Cc: stable@dpdk.org
Reported-by: Andrey Nikolaev <gentoorion@gmail.com>
Suggested-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Signed-off-by: Yipeng Wang <yipeng1.wang@intel.com>
Acked-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
Acked-by: John McNamara <john.mcnamara@intel.com>