* 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.
