Clarify JTAG connector section. (#782)

* Clarify JTAG connector section.

Make it more clear that MIPI-10 should be sufficient for everyone,
mostly by clearly putting MIPI-20 into an alternate connector section.

Expand just a tiny bit on cJTAG.

Update MIPI references to the current version of that document, and
mention the pinouts using the same names used there.

* Clarify what nTRST is for.

* Clearly label MIPI-10 as the Recommended JTAG Connector.

* more esoteric signals -> legacy JTAG signals

* nRESET is open drain.

* Unnecessary pins may be left unconnected

* Describe EXT pin.

* RTCK and TRIGOUT should be GND if not used.

* Clarify that nTRST_PD and RTCK are undesirable.

Reorganize the trigger signal table into Essential, Recommended,
Advanced, Specialized, and Legacy signals.