Review and update docs for `rustc_infer::infer::lattice`

[pierwill]

The docs for rustc_infer::infer::lattice (source here) refer to a file called infer.rs.

Currently, we only have two files with that name:

• ./compiler/rustc_trait_selection/src/infer.rs
• ./compiler/rustc_typeck/src/outlives/implicit_infer.rs

Where is thelattice code referred to here?

Perhaps we should describe the implementation here more instead of just linking to file.

[pierwill]

@rustbot claim

[pierwill]

• Document rustc_infer::infer::lattice::LatticeDir.
• Document rustc_infer::infer::lattice::super_lattice_tys.
• Document rustc_infer::infer::glb and rustc_infer::infer::lub.