Improve the documentation of Dhall.Import

[sjakobi]

This module contains a lot of tricky logic that would be easier to stomach with good documentation.

Bits of the current documentation seem outdated to me or I might be misunderstanding them:

dhall-haskell/dhall/src/Dhall/Import.hs

Lines 236 to 237 in 8c8ea2a

	common unintentional violations. To do this, Dhall enforces that a
	non-local import may not reference a local import.

A good high-level description of the caching strategy (both "semantic" and "semi-semantic") would also be good. The link to #1098 is helpful, but with issues you never know how many comments you'll have to read, too, and whether it might contain outdated information.