document crate-relative paths, absolute paths, path contexts, etc.

[jesse99]

The new behavior needs to be documented, including the semantics of the self and super keywords.

[catamorphism]

Nominating for milestone 1, well-defined.

[thestinger]

We're still missing good documenting on resolve.

[graydon]

accepted for well-defined milestone

[alexcrichton]

self, super, and crate-relative paths being required for use statements are now documented in the manual starting at 35ec01a

[emberian]

There are some examples, but still no particularly clear semantics defined, afaict.

[brson]

Nominating for removal from milestone.