src/source/advanced.rst: Fix serial port documentation

• the serial connector is labeled J16 and not J16
• the connector is not labeled S1 and S2
• explain how the pins are numbered
• fix table associating pins to RX, TX and GND of S1 and S2

Link: https://www.cs.unb.ca/~bremner/blog/posts/hibernate-pocket-3/ Thanks: David Bremner bremner@debian.org