auth: special-memory-usage, real-memory-usage: improve docs

[aj-gh]

• Program: Authoritative (+Recursor, dnsdist for docs)
• Issue type: "bit of both"

Short description

330dcb5c61d20d26c7d782c9eb7bd5872664e10a introduces special-memory-usage which seems to lack a bit of documentation, e.g. in https://github.com/PowerDNS/pdns/blob/master/docs/performance.rst (which also seems to lack real-memory-usage). Additionally even though it's described as expensive it's still returned by default in pdns-auth - should this be the case?

Environment

• Operating system: all
• Software version: 4.2.0-rc1
• Software source: 4.2.0-rc1 source package

Steps to reproduce

pdns_control show "*"

[Habbie]

Additionally even though it's described as expensive it's still returned by default in pdns-auth - should this be the case?

Fixed in #7887