Doc update regarding source files not being UTF-8

[Vlad Dumitrescu]

Hi!

I think it would be a good idea if the introduction chapter in the
reference manual, in the "Character set" section, would note
specifically that even the comments in the source code can't contain
UTF-8 characters because the lexical scanner doesn't understand them.
Well, they might contain them but will be garbled if processed by any
source tool (like for example syntax_tools) because Latin-1 and UTF-8
aren't compatible.

The Eclipse editor will even refuse to save files with UTF-8 (so that
they don't get garbled). If you think that it's a good idea to mention
that too, please do so.

I couldn't come up with a nice way to express this clearly, so there
is no patch.

best regards,
Vlad