Update document for maximum packet size in socket hal

If LE buffer size is zero, buffers returned by Read_Buffer_Size are shared between Classic and LE. Then, hsot stack uses ACL data packet length for MPS.

Bug: 342012881
Flag: EXEMPT, document only
Test: m android.hardware.bluetooth.socket-update-api
Change-Id: I594bec5586aa748e3c6b4d5c964abb28212ad412