Can't document static / constant variables?

```python
from nbdev import show_doc

# this gets ignored
MY_CONST = 'hello' # this gets ignored

# this shows hello not MY_CONST
showdoc(MY_CONST)
```

Reproduce the notebook example using nbdev.show_doc and the MY_CONST assignment, then inspect the show_doc entry point and its handling of constant values. Done means the constant is documented with its name and intended value rather than being ignored or displayed only as hello.

Written by the indexing model from the issue text.