cmd ref: update gc --all-commits desc

per #1023 (comment)

content/docs/command-reference/gc.md

@@ -61,10 +61,11 @@ The default remote is cleaned (see `dvc config core.remote`) unless the
   commits as well as the workspace (implies `-w`). Useful for keeping all the
   data used in the entire existing commit history of the project.
 
-  One of the use cases for this option is to safely delete all temporary data
-  DVC cached when `dvc run` and/or `dvc repro` were run without committing
-  changes to DVC-files (thus potentially caching data that is not referenced
-  from workspace or Git commits).
+  A use case for this option is to safely delete all temporary data `dvc run`
+  and/or `dvc repro` cache when used without committing changes (see the `-O` or
+  `-M`, and `--no-commit` options in those commands). In that scenario, data
+  that is never referenced from the workspace or from any Git commit can still
+  be stored in the project's cache).
 
 - `-p <paths>`, `--projects <paths>` - if a single remote or a single cache is
   shared among different projects (e.g. a configuration like the one described