Define some more terms/acronyms. (#610)

* Define some more terms/acronyms.

Many acronymos contributed by Bruce Ableidinger.

* CSR = Control and Status Register

Co-authored-by: Paul Donahue <48959409+pdonahue-ventana@users.noreply.github.com>

* IR = JTAG IR

Co-authored-by: Megan Wachs <megan@sifive.com>

* DR = JTAG DR

Co-authored-by: Megan Wachs <megan@sifive.com>

* XLEN is hart-specific

Co-authored-by: Megan Wachs <megan@sifive.com>

* Clarify virtual address

Co-authored-by: Megan Wachs <megan@sifive.com>

* Review feedback.

Removed a bunch of terminology entries, and clarified several others.

* Add TM.

* Slight tweaks based on review.

* Use "hardware platform" consistently.

I hope.

* Simpler language

Co-authored-by: Paul Donahue <48959409+pdonahue-ventana@users.noreply.github.com>

* Alphabetize list.

Co-authored-by: Paul Donahue <48959409+pdonahue-ventana@users.noreply.github.com>
Co-authored-by: Megan Wachs <megan@sifive.com>