doc: Correct msgq data item size discrepancy

Updates the data passing summary table to indicate that the size of
a message queue data item must be a multiple of its data alignment.
This brings the documentation in both the summary table and the
message queue documentation into alignment.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>

doc/kernel/services/index.rst

@@ -59,7 +59,7 @@ Object            Bidirectional?      Data structure         Data item size
 FIFO              No                  Queue                  Arbitrary [1]              4 B [2]   Yes [3]            Yes             N/A
 LIFO              No                  Queue                  Arbitrary [1]              4 B [2]   Yes [3]            Yes             N/A
 Stack             No                  Array                  Word                          Word   Yes [3]            Yes             Undefined behavior
-Message queue     No                  Ring buffer            Power of two          Power of two   Yes [3]            Yes             Pend thread or return -errno
+Message queue     No                  Ring buffer            Arbitrary [6]         Power of two   Yes [3]            Yes             Pend thread or return -errno
 Mailbox           Yes                 Queue                  Arbitrary [1]            Arbitrary   No                 No              N/A
 Pipe              No                  Ring buffer [4]        Arbitrary                Arbitrary   Yes [5]            Yes [5]         Pend thread or return -errno
 ===============   ==============      ===================    ==============      ==============   =================  ==============  ===============================
@@ -79,6 +79,8 @@ argument.
 [5] ISRS can send and/or receive only when passing K_NO_WAIT as the
 timeout argument.
 
+[6] Data item size must be a multiple of the data alignment.
+
 .. toctree::
    :maxdepth: 1