Put line numbers in vimdoc exceptions.

[Soares]

module.py Module.Merge may be a good place to start.

In fact, vimdoc exceptions should be made more intelligent across the board. Line numbers are a good place to start, though.

[dbarnett]

Also warnings.

The fact that we don't have line number for warnings combined with the fact that we show a few spurious warnings (e.g., for functions with multiple @usages) means that it's already way too tempting to just ignore warnings and miss actual problems. I just discovered several legitimate warnings we've been ignoring in maktaba: google/maktaba#69.