dev docs: key-management.md updates and clarifications
This partially addresses the comments and questions from #853 (closed)
More specifically, this MR:
- renames
KeyIdentity
toKeySpecifier
- describes in more what the C Tor store config looks like (I've explained how it's going to work for hidden services)
- explains how, unlike the Arti store, the C Tor store doesn't have a single root (it consists of several directories, the common ancestor of which might well be
/
) - contains some other minor cleanups/clarifications
cc @Diziet