Better section documentation

[SmileyChris]

From @adiroiban comments in #603

Especially the part from Section Path Behaviour is confusing.

I think that it needs better example.

For example

If directory is not set, “newsfragments” is added to the end of each path. For example, with the above sections, the paths would be:

Which directory , I guess that this is [tool.towncrier] -> directory

I agree the docs should be better. Probably a top level file rather than a section within the configuration docs would be actually better.

Also, the doc example I think I wrote a long time ago because the current behaviour wasn't explained at all (and it's confusing behaviour!)