Add `enter` and `exit` terms to glossary

[wooorm]

Subject of the feature

Other that the different ways of traversing a tree, as raised in GH-22 and resolved in GH-23, the terms enter and exit are also often used when discussing tree traversal. These should be added to the glossary as well.

Problem

Some form of state is often mutated when entering or exiting a node by unist utilities. Describing these terms here means they can be linked to from other docs to clarify them in a single place.

Expected behaviour

Both should be added to the glossary.

Alternatives

The alternatives (status quo) are shown in problem above.