Improve explanation of system macro address allocation

[toddjonker]

          This aligns with my understanding. However, I was confused by the prior statement that:

to a large degree, it's as if .... macro_table clauses all have $ion as their implicit first element

as that would load those into the one-byte-invocations space. (Maybe that nuance is what "to a large degree" was referring to?)

Originally posted by @zslayton in https://github.com/amazon-ion/ion-docs/pull/258#discussion_r1223534644