Improve activator documentation

[adamchainz]

I wanted to point to a reference for VIRTUAL_ENV_DISABLE_PROMPT. It seems the existing documentation section for it was lost in #1519. Whilst restoring it, I tried to wordsmith the surrounding area of documentation to make it flow a little easier - less bracketed phrases, more consistent terminology, and more explanation of the prompt modification.

Thanks for contributing, make sure you address all the checklists (for details on how see development documentation)

• ran the linter to address style issues (tox -e fix_lint)
• wrote descriptive pull request text
• [n/a] ensured there are test(s) validating the fix
• [n/a] added news fragment in docs/changelog folder
• updated/extended the documentation

[gaborbernat]

Can you fix the CI please.

[adamchainz]

Thanks!