Bluetooth: Mesh: Kconfig: Tweak RX SDU value

Update the RX SDU Kconfig value to something that reflects better
current use cases and doesn't waste memory needlessly. Also lower the
minimum to two segments, since while many samples need three for their
composition data (typically the biggest transferred payload), it's
possible to have a very simple node whose composition fits in two
segments.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>

subsys/bluetooth/host/mesh/Kconfig

@@ -206,12 +206,15 @@ config BT_MESH_RX_SEG_MSG_COUNT
 
 config BT_MESH_RX_SDU_MAX
 	int "Maximum incoming Upper Transport Access PDU length"
-	default 384
-	range 36 384
-	help
-	  Maximum incoming Upper Transport Access PDU length. Leave this
-	  to the default value, unless you really need to optimize memory
-	  usage.
+	default 72
+	range 24 384
+	help
+	  Maximum incoming Upper Transport Access PDU length. This
+	  determines also how many segments incoming segmented messages
+	  can have. Each segment can contain 12 bytes, so this value should
+	  be set to a multiple of 12 to avoid wasted memory. The minimum
+	  requirement is 2 segments (24 bytes) whereas the maximum supported
+	  by the Mesh specification is 32 segments (384 bytes).
 
 config BT_MESH_TX_SEG_MAX
 	int "Maximum number of segments in outgoing messages"