DOC/MINOR: list: fixing MT_LIST_LOCK_ELT macro documentation

DOC/MINOR: list: fixing MT_LIST_LOCK_ELT macro documentation

MT_LIST_LOCK_ELT macro was documented with an ambiguous
usage restriction, implying that concurrent list deletion
was not supported.

But it seems that either the code has evolved, or the comment is
wrong because the locking behavior implemented here is exactly
the same one used in MT_LIST_DELETE, and no such restriction is
described for MT_LIST_DELETE.

I made some tests to make sure concurrent MT_LIST_DELETE (or deletion
from mt_list_for_each_entry_safe) don't cause unexepected results.

At the present time, this macro is not used, this fix only
targets upcoming developments that might rely on this.

No backport needed.