Addressed inaccuracies in performance docs

* Move the discussion of CachedStaticFilesStorage to the section about
  HTTP. It's really about client-side caching. It doesn't fit with the
  caching utilities from django.utils.functional.

* Tone down the warning against Pypy as per Alex' feedback. It's a valid
  choice for advanced users who are comfortable using a non-standard
  stack.

* Generally reworded the 'Using different versions of available software'
  section.

* Some other minor adjustments to the document.