Add a note about Universal Assembly Language (UAL) syntax

[exelotl]

As mentioned by @AntonioND in #91 (comment) regarding the Quick Reference sheet that's linked in a few places:

ARM is trying to get the Unified Assembler Language (UAL, as seen in the file name QRC0001_UAL.pdf) adopted by ARM developers. Personally, I think UAL is a good idea (with the problem that it has come way too late) so I would just use that link. However, I think this would require some note in Tonc because most code out there is legacy non-UAL code, and it's a good idea to mention this because it's non-obvious.

So we should probably mention this in the asm chapter somewhere.