Merge pull request #1250 from jedevc/prune-all-help

docs: add correct definition of prune --all flag

docs/reference/buildx_prune.md

@@ -11,7 +11,7 @@ Remove build cache
 
 | Name | Type | Default | Description |
 | --- | --- | --- | --- |
-| `-a`, `--all` |  |  | Remove all unused images, not just dangling ones |
+| `-a`, `--all` |  |  | Include internal/frontend images |
 | [`--builder`](#builder) | `string` |  | Override the configured builder instance |
 | `--filter` | `filter` |  | Provide filter values (e.g., `until=24h`) |
 | `-f`, `--force` |  |  | Do not prompt for confirmation |
@@ -21,6 +21,26 @@ Remove build cache
 
 <!---MARKER_GEN_END-->
 
+## Description
+
+Clears the build cache of the selected builder.
+
+You can finely control what cache data is kept using:
+
+- The `--filter=until=<duration>` flag to keep images that have been used in
+  the last `<duration>` time.
+
+  `<duration>` is a duration string, e.g. `24h` or `2h30m`, with allowable
+  units of `(h)ours`, `(m)inutes` and `(s)econds`.
+
+- The `--keep-storage=<size>` flag to keep `<size>` bytes of data in the cache.
+
+  `<size>` is a human-readable memory string, e.g. `128mb`, `2gb`, etc. Units
+  are case-insensitive.
+
+- The `--all` flag to allow clearing internal helper images and frontend images
+  set using the `#syntax=` directive or the `BUILDKIT_SYNTAX` build argument.
+
 ## Examples
 
 ### <a name="builder"></a> Override the configured builder instance (--builder)