Clarify behaviour of plugin with no version/tag/commit specified

The plugin section of the documentation doesn't appear to define the behaviour of a plugin defined without a pinned hash.

If I define a plugin like this in a pipeline:
```
plugins:
- myplugin:
...
```
it appears that the HEAD of the default branch is checked out.

If a new commit is subsequently pushed to the plugin's repo, running the pipeline again does not appear to check out the latest commit but instead the previously checked out one.

If this is expected behaviour, it might be clearer to specify it in the docs.