Document "Spaces between label and colons" in rgbasm-old.5

[Rangi42]

I thought this would count as a mere syntax bugfix, but it's turned up a couple of times (e.g. in pokegold-spaceworld and some project used to have a Label: :), so may as well document it.

(Be sure to check when we added :: as its own token.)

Also, unrelated, but the heading "LDHL, SP, e8" should be "LDHL SP, e8".

[Rangi42]

Heck, maybe we should document add r16 and add sp for add hl, r16 and add hl, sp too. Or even = for set as an instruction. The line between "fixed bug" and "removed feature" is blurry, and people do whatever silly thing is technically possible. :P