Show the name of the documented symbol

This shows the name of the documented symbol in the test output:
```
[info] MainDoctest:
[info] Main.scala:4: f
[info] - Main.scala:8
[info] - Main.scala:11
[info] Main.scala:18: xyz
[info] - Main.scala:25
[info] Main.scala:31: abc
[info] - Main.scala:36
[info] Main.scala:42: list
[info] - Main.scala:44
```

Instead of only filenames and linenumbers as it is now:
```
[info] MainDoctest:
[info] Main.scala:4
[info] - Main.scala:8
[info] - Main.scala:11
[info] Main.scala:18
[info] - Main.scala:25
[info] Main.scala:31
[info] - Main.scala:36
[info] Main.scala:42
[info] - Main.scala:44
```
I'm not sure whether `parsed.nameChain` can be empty, I think `.lastOption.map(_.toString).getOrElse("")` is safer.