improve docstrings of makeUniversal templates, allow extensions

[Vindaar]

Each makeUniversal call accepts a docSuffix argument, which can be used to provide an additional piece of text to be added to that calls doc.

@AngelEzquerra: Feel free to adjust any of the makeUniversal calls in ufunc.nim and give some extra information. It will just be added as a paragraph to the autogenerated text.

Note: The previous comment

# Note, the makeUniversal/Local documentation gets duplicated in docs at each template call
# And shouldn't use ##

in the code seems to be outdated. This does not seem to happen anymore. Therefore I've made the doc string of the two templates regular ## doc comments.

[AngelEzquerra]

I just tested it and it works great! Thank you!