Improve pruning CLI documentation (#4810)

Closes: #4801

@andreclaro I hope this makes it more clear from the docs directly.

substrate/client/cli/src/params/pruning_params.rs

@@ -17,7 +17,7 @@
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
 use crate::error;
-use clap::Args;
+use clap::{builder::PossibleValue, Args, ValueEnum};
 use sc_service::{BlocksPruning, PruningMode};
 
 /// Parameters to define the pruning mode
@@ -29,29 +29,24 @@ pub struct PruningParams {
 	/// should be pruned (ie, removed) from the database.
 	/// This setting can only be set on the first creation of the database. Every subsequent run
 	/// will load the pruning mode from the database and will error if the stored mode doesn't
-	/// match this CLI value. It is fine to drop this CLI flag for subsequent runs.
-	/// Possible values:
-	///  - archive: Keep the state of all blocks.
-	///  - 'archive-canonical' Keep only the state of finalized blocks.
-	///  - number Keep the state of the last number of finalized blocks.
+	/// match this CLI value. It is fine to drop this CLI flag for subsequent runs. The only
+	/// exception is that `<NUMBER>` can change between subsequent runs (increasing it will not
+	/// lead to restoring pruned state).
+	///
 	/// [default: 256]
-	#[arg(alias = "pruning", long, value_name = "PRUNING_MODE")]
+	#[arg(alias = "pruning", long, value_name = "PRUNING_MODE", value_enum)]
 	pub state_pruning: Option<DatabasePruningMode>,
 
 	/// Specify the blocks pruning mode.
 	///
 	/// This mode specifies when the block's body (including justifications)
 	/// should be pruned (ie, removed) from the database.
-	/// Possible values:
-	///  - 'archive' Keep all blocks.
-	///  - 'archive-canonical' Keep only finalized blocks.
-	///  - number
-	///  Keep the last `number` of finalized blocks.
 	#[arg(
 		alias = "keep-blocks",
 		long,
 		value_name = "PRUNING_MODE",
-		default_value = "archive-canonical"
+		default_value_t = DatabasePruningMode::ArchiveCanonical,
+		value_enum
 	)]
 	pub blocks_pruning: DatabasePruningMode,
 }
@@ -83,6 +78,26 @@ pub enum DatabasePruningMode {
 	Custom(u32),
 }
 
+impl ValueEnum for DatabasePruningMode {
+	fn value_variants<'a>() -> &'a [Self] {
+		&[Self::Archive, Self::ArchiveCanonical, Self::Custom(0)]
+	}
+
+	fn to_possible_value(&self) -> Option<clap::builder::PossibleValue> {
+		Some(match self {
+			Self::Archive => PossibleValue::new("archive").help("Keep the data of all blocks."),
+			Self::ArchiveCanonical => PossibleValue::new("archive-canonical")
+				.help("Keep only the data of finalized blocks."),
+			Self::Custom(_) => PossibleValue::new("<NUMBER>")
+				.help("Keep the data of the last <NUMBER> of finalized blocks."),
+		})
+	}
+
+	fn from_str(input: &str, _: bool) -> Result<Self, String> {
+		<Self as std::str::FromStr>::from_str(input)
+	}
+}
+
 impl std::str::FromStr for DatabasePruningMode {
 	type Err = String;