docs: Clarify when graph methods return None? (#491)
docs: Clarify when graph methods return None? (#491)

This is a docs change, and should not affect any functionality.

While looking about the API, I didn't quite understand why I had to `unwrap()` every time I wanted to access an edge or a node. I understand now it's because that weight or node might not exist, but that was only clear to me after looking at the source code.

This change adds a line to the relevant docstrings that explain when a `Some()` will be returned vs when a `None` will be returned.

The docstring for `remove_node` already had a similar line in it:
```
/// Remove `a` from the graph if it exists, and return its weight.
/// If it doesn't exist in the graph, return `None`.
```
So I made sure the lines I added were phrased similarly to keep the code base consistent, like 
```
/// If ____ doesn't exist in the graph, return `None`.
```
petgraph/petgraph