DOC: config: document logformat item naming and typecasting features

DOC: config: document logformat item naming and typecasting features

The ability to give a name to a logformat_node (known as logformat item in
the documentation) implemented in 2ed6068f2a ("MINOR: log: custom name for
logformat node") wasn't documented.

The same goes for the ability to force the logformat_node's output type to
a specific type implemented in 1448478d62 ("MINOR: log: explicit
typecasting for logformat nodes")

Let's quickly describe such new usages at the start of the custom log
format section.