docs: clarify GETTABLE empty-reply and error semantics

[tobez]

Fixes #1.

The manual said GETTABLE's possible per-oid errors are "the same as in
GET", which set the expectation that walking a non-existent table
returns a "no-such-object" error the way GET does. It doesn't: the
underlying get-next/get-bulk queries cannot distinguish an empty table
from one the agent does not implement, so both produce a successful
reply with an empty array.

Manual changes (man page regenerated with pandoc):

• state that "no-such-object" / "no-such-instance" never occur in
  GETTABLE replies;
• explain that an empty reply array is a successful reply covering both
  the empty-table and non-existent-table cases;
• relabel example reply 2, which previously showed the empty reply only
  for the special case of walking a leaf oid.

Documentation only; no behavior change.