[tldoc] tweaks to manual

[Karl Berry]

Hi TL translators,

We've made a few final (hopefully) tweaks to the English manual:
- the context section is mostly rewritten;
- small mentions of the \code{V} command in the text installer and
  another tweak or two in the portable section.

If you haven't started the translation updates yet, please do.
I'd like to have them committed by June 20 (10 days from now)
at the latest.

Thanks,
Karl