[Accessibility] TTS API interface description + reqs update

[Hynek Hanke]

> Changes + additions

I forgot to mention that in the interface description,
I'm using wchar_t and UTF-32, despite the requirements
document mentioning UTF-8.

I think the requirement document should be changed on this
point. The important thing is that Unicode is used.

Using UTF-8 (variable width characters) is quite impractical,
especially as event and index marker positions are given
in characters, not bytes.

UTF-8 and UTF-32 can be freely converted where necessary.

With regards,
Hynek Hanke