confusion between node and client/gateway in configuration.rst

[zooko]

• The ​description of "web.port" assumes that the node is running a client/gateway. This should be amended to say something like "providing node status and, if the node is running a storage client a.k.a. gateway service, providing filesystem access as defined in webapi.rst <frontends/webapi.rst>_".

• The docs in docs/configuration.rst​ should be edited a little to explain that the thing called a "Tahoe-LAFS storage client" is also sometimes called a "Tahoe-LAFS gateway".

• The ​docs of the node section should include mention of a storage server and helper as kinds of nodes, in addition to storage client and introducer.

In addition, I think the following is a related issue:

• As far as I can tell, the tempdir field from ​the "node" section of the configuration file belongs in ​the "client" section, because a temp directory for uploading files is only used for clients/gateways.

But since fixing that will require an actual code change and not just a documentation change I'll put it in a separate ticket.