Document how to upgrade old syntax in a couple of entries of the guide

[AntonioND]

It would be useful to have examples in the following two entries (and other entries I may have missed, but those are the warnings I saw yesterday when building my projects):

https://rgbds.gbdev.io/docs/v1.0.1/rgbasm-old.5#Treating_strings_as_numbers

https://rgbds.gbdev.io/docs/v1.0.1/rgbasm-old.5#Treating_multi-character_strings_as_numbers

[Rangi42]

Good idea. Something like:

For example, instead of this string literal:

    ld a, "R"

Use this character literal:

    ld a, 'R'

And instead of this string expression:

    cp STRUPR("\1")

Use this numeric expression:

    cp CHARVAL(STRUPR("\1"))

And:

For example, instead of this multi-character string:

    charmap "X", $12
    charmap "Y", $34
    ld bc, "XY"

Either combine the individual characters:

    charmap "X", $12
    charmap "Y", $34
    ld bc, 'X' << 8 | 'Y'

Or use this single character:

    charmap "<XY>", $1234
    ld bc, '<XY>'

Or use this multi-value CHARMAP:

    charmap "<XY>", $12, $34
    ld bc, CHARVAL("<XY>", 0) << 8 | CHARVAL("<XY>", 1)

(That example for multi-value charmaps doesn't really demonstrate their real usefulness. The problem with single-value charmaps is that you can't tell their width -- sure charmap "<XY>", $1234 is obviously meant to be used as a two-byte word, but there's no difference between charmap "B", $01 and charmap "<AB>", $0001. Whereas with charmap "<AB>", $00, $01, you can measure its width with CHARSIZE("<AB>"). This is mainly useful for macros, like pret poketcg uses.)

[AntonioND]

Yep, works for me!