pioasm: document predefined constants zero and one

[magy00]

Resubmission of #1862:

Error messages are not obvious. Maybe allow zero and one to be assigned the values 0 and 1, resp., repeatedly.

test.pio:

.program test
.origin 0
_0:
zero:
        jmp skip
_1:
one:
        mov pins, ~pins
skip:

        nop [1]

        out pc, 1       // effectively jmp _0 or jmp _1

$ ~/.pico-sdk/tools/2.0.0/pioasm/pioasm test.pio
test.pio:4.1-4: syntax error, unexpected integer
    4 | zero:
      | ^~~~
test.pio:7.1-3: syntax error, unexpected integer
    7 | one:
      | ^~~

Similar for (obvious?) labels like clear, wait, status, etc., are mistaken as keywords:

test.pio:

.program test
clear:
wait:
status:
block:

$ ~/.pico-sdk/tools/2.0.0/pioasm/pioasm test.pio
test.pio:2.1-5: syntax error, unexpected clear
    2 | clear:
      | ^~~~~