Fix Num.+ Scaladoc

Change Num.+ Scaladoc to state that this is not a growing
addition. Note that this is problematic either way as this macro is
resolved to an abstract method. Classes implementing this typeclass
are technically free to violate what we put in the Scaladoc here.

h/t @kammoh

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