Improve documentation

[akesterson]

There have been several emails and bugs filed about dpath's documentation. Apparently there are several users who find it confusing, and even the maintainers have found the documentation frustrating at times.

Let's please link all documentation-related incidents here, so we can get them all knocked out at once.

• provide example of list of dictionaries and filtering by intermediate key #42 - The documentation for .values() is right there in the README, but somehow the user missed it. How can we make that more obvious?
• Document how to seek keys with more than one letter #15 - the documentation implies to some people that keys must be 1 character in length.
• Document globbing syntax #38 - Document the globbing syntax

[akesterson]

Some of these updates are bugs in the docs, some are enhancements, labeling as both bug and enhancement.

[abarnert]

Note that in #15 the answer incorrectly says [a..z] when it should say [a-z], and also doesn't get across that the rules are actually pretty complicated (consider what [] or []a] mean). So, I think making it very clear that they mean what fnmatch mean is pretty important. (The alternative would be to full document fnmatch syntax—which is especially fun given that there have been a couple of minor changes from 2.6 to 3.7 that you'd have to document.)

Also #80 probably requires a doc change whether it's fixed or not. Obviously replacing fnmatch with not-quite-fnmatch means you can no longer just defer to it for documentation. But leaving it alone means you probably need to at least document the fnmatch way of escaping (with a single-character glob class), and maybe point out why it can't be used for separators.

[akesterson]

Blocked by #136