Update docs on dontTouch as optimization barrier (#2015)

Change the Scaladoc for the dontTouch utility.  Indicate that this is an
_optimization barrier_ and not just a guarantee that the signal won't be
removed.  The optimization barrier interpretation is the current
implementation in the Scala FIRRTL Compiler.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>