block label differs in implementation and documentation

[wilhelmmailaender]

In the documentation the label is optional for the server, api, spa and files block.

But the implementation differs. The label is required for server, and not allowed for api, spa and files. Other blocks may be also differ between implementation and documentation.

The implementation should make the label optional for all blocks, as it is in the documentation.

[malud]

We should adapt the docs for now. The server label is required and not designated for files spa and api (currently).

/cc @filex