reflect,doc: use "the" instead of "a" in IsZero docs
There is a subtle distinction between a value
*being* the zero value vs being *equal to* the zero value.
This was discussed at length in #31450.
Using "a zero value" in the docs suggests that there may
be more than zero value. That is possible on the "equal to
zero value" reading, but not the "is zero" reading that we
selected for the semantics of IsZero.
This change attempts to prevent any confusion on this front by
switching to "the zero value" in the documentation.
And while we're here, eliminate a double-space.
(Darn macbook keyboards.)