Closes #139

Converted "verbatim" to AsciiDoc tables where it is appropriate.

Unify vl=0 and vstart >= vl rules

Clarify that vsetvl resets vstart to 0, too

Closes #127

Currently, vl=0 is a special case in the spec: elements past vl are left
intact in this case.  vstart >= vl (when vl > 0) is sort of a special case
itself: no elements below vl are updated, but elements past vl are zeroed.

The proposal is to unfiy these cases, and leave the entire vector intact
when vstart >= vl.  Or, put another way, the elements past vl are zeroed if
and only if some other element in the vector needs to be written.

Clarifies that same VLMAX setting gives same vl for a given AVL.
Fixes bug in mixed-width code sample.

h/t Filip Moc

Closes #121

Consensus after 3/23/2019 TG meeting.

Revert "formatted document for better HTML / PDF rendering"

formatted document for better HTML / PDF rendering