Enhance documentation on BuiltinString and BuiltinByteString usage in Plinth

[Unisay]

Closes https://github.com/IntersectMBO/plutus-private/issues/1575

Rendered:

(there is a glitch in the middle caused by my screenshot tool, actual documentation is ok)

[zliu41]

I don't remember if this is written anywhere, but in doc files we write each sentence on a separate line. This makes it easier to comment and see the diff.

[Unisay]

I don't remember if this is written anywhere, but in doc files we write each sentence on a separate line. This makes it easier to comment and see the diff.

Done: edited the doc to start each sentence from a new line.

[zliu41]

I think the example can be as simple as

ghci> unpack ("\256" :: ByteString)
[0]

But yours is fine too.