doc/ref: add spec for bounds
doc/ref: add spec for bounds

Bound is an easy spec (piggy backed on comparisons
and allows expressing things that are currently impossible
to express. Once added, we could seriously consider removing
ranges.

open ranges:
a: >=1 & <5.0 // open range

positive: >0 // allows 1e-100, but not 0

exclude values:
b: null | a1 | … | an
b: !=null

nonZero: !=0

Change-Id: I4c461b85849a347e423570c364e382650db20069

cuelang/cue