man: further file-hierarchy *Directory= improvements

Follow-up to @poettering’s comments in #7723:

- Slightly expand on the difference between using tmpfiles.d and service
  directives
- Mention CacheDirectory=
- Mention LogsDirectory=
- Abbreviate and unify some later descriptions

ConfigDirectory= is not mentioned, since it does not support the
functionality mentioned in the manpage which tmpfiles.d provides:
copying or symlinking default configuration from /usr/share/factory. And
the user package variable file locations don’t mention the directives
because in user units the service can always create the directories
itself (whereas in system units lesser-privileged services lack
permission to create them).