docs: run: update --no-exec description (#1225)

* docs: run: update --no-exec description

Per iterative/dvc#3729

* Update run.md

content/docs/command-reference/run.md

@@ -153,8 +153,10 @@ data pipeline (e.g. random numbers, time functions, hardware dependency, etc.)
 - `--no-exec` - create a stage file, but do not execute the `command` defined in
   it, nor track dependencies or outputs with DVC. In the DVC-file contents, the
   file hash values will be empty; They will be populated the next time this
-  stage is actually executed. This is useful if, for example, you need to build
-  a pipeline (dependency graph) first, and then run it all at once.
+  stage is actually executed. DVC will also add your outputs to `.gitignore`,
+  same as it would do without `--no-exec`. This is useful if, for example, you
+  need to build a pipeline (dependency graph) first, and then run it all at
+  once.
 
 - `--overwrite-dvcfile` - overwrite an existing DVC-file (with file name
   determined by the logic described in the `-f` option) without asking for