memory: document existing diagnostics and debugging features
memory: document existing diagnostics and debugging features

Although the individual classes and methods are documented, they are
currently hard to discover as there is no summary of the individual
features and the various knobs that belongs to them. This patch adds
exactly this.

cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,dreamsxin/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,dreamsxin/seastar