Part of #33.
The v6.S listing in the hello_world chapter introduces the PUSH_P,
PUSH_R, POP_P and POP_R macros without explicitly telling the reader
what the _P and _R suffixes stand for. For a reader reading the
chapter in order, the mapping is derivable: the book established
stp = "store pair" about 260 lines earlier, and the _R suffix falls
out by parallel with str. But a reader who jumped straight to v6.S
without reading the stp explanation (as Atcold did in issue #33) has
no such anchor and is left guessing.
One sentence at the "The push and pop macros simply save typing"
paragraph closes the gap: it names the _P / _R suffixes, connects
them to the already-taught stp / str convention, and costs a skimmer
nothing. The macro names themselves are unchanged; they were already
fine for an in-order reader, and renaming would have churned every
.S file in the book.
