doc: Improve section on unattended key generation.
* doc/gpg.texi: Improve the subsection on unattended key generation by suggesting the quick key manipulation interface as an alternative, and by suggesting alternatives to '%pubring' and '%secring'. Simplify examples accordingly.
- GnuPG-bug-id: T2400
- Signed-off-by: Justus Winter <justus@g10code.com>