Fix outdated commentary about LR/SC and context switching

Closes riscv#193

Remove misleading note about AMOSWAP elision

The note implied that it's legal to elide silent stores.  This is not true
with respect to the consistency model.

Explicitly mention that FS may be imprecise (riscv#192)

Closes riscv#147

Switch to active voice (riscv#191)

Also fix: the pseudoinstruction use absolute addressing, the base
instructions are pc-relative.

Switch to active voice (riscv#191)

Also fix: the pseudoinstruction use absolute addressing, the base
instructions are pc-relative.

FENCE.TSO is not a pseudoinstruction

fix typo

Clarify sign-extension of offsets in assembly manual

Closes riscv#177

Expand divide-by-0 table to cover the *W instructions

h/t Alexandre Joannou

Add Alexandre Joannou to contributors