cli: Improve pruning documentation (#12819)

* cli: Improve pruning documentation

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* cli: Keep `finalized` notation and remove `canonical` one

* cli: Fix cargo doc

* cli: `PruningModeClap` IR enum

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* cli: Convert PruningModeClap into pruning modes

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* cli: Use `PruningModeClap`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* cli: Rename to `DatabasePruningMode`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* cli: Implement `FromStr` instead of `clap::ValueEnum`

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Update client/cli/src/params/pruning_params.rs

Co-authored-by: Bastian Köcher <git@kchr.de>

* Fix clippy

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* cli: Add option documentation back

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Apply suggestions from code review

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>