document impassability

[tomprogers]

Describe in the API docs how to use the options to make a node impassable.

Consider this grid:

┌───┐
│ A │
├───┤
│   │
├───┤
│ B │
└───┘

Where:

• empty cells should be impassable -- walls, mountains, lava, force-field, whatever
• the caller wants to ask for a path from cell A to cell B, which ought to be impossible

It appears this cannot be accomplished by the distance function, e.g. by returning Number.POSITIVE_INFINITY as the cost for entering an impassable node. Instead, it seems like the neighbor function must choose to omit nodes which should be treated as impassable.