Document cache design in `misc/design-philosophies`

[jamesp]

I initially found the design of the cache backend classes confusing, especially with respect to references to Memcached in django.core.cache.backends.base.BaseCache.validate_key() . I found that this was added a year ago in ticket #6447, so I figured there was a good reason but it was not explained anywhere I could find.

I discussed this with a couple core developers and hopefully the attached patch will be suitable.

[Gabriel Hurley]

Not having been privy to the discussion on the issue, I'm a little unclear on what problem the last section on Explicit vs. Implicit is trying to address. While I agree with the sentiment I'm left with the question "why would it be tempting to do the opposite?"

Otherwise the patch looks good. If you could say a few words on the meaning of that last section I'll wrap it up and commit it.

[Tim Graham]

Since we can't get clarification on the "explicit vs. implicit" section, I'm going to commit the rest of this.

[Tim Graham <timograham@…>]

In dd98ccd4821f71543a5489d421ea2dee28901f17:

Fixed #16785 -- Documented some of the design decisions of the cache framework.

Thanks jamesp for the draft patch.