params: document --targets option (#2031)

* params: document --targets option

* Update content/docs/command-reference/params/diff.md

Co-authored-by: Jorge Orpinel <jorgeorpinel@users.noreply.github.com>

* Restyled by prettier (#2042)

Co-authored-by: Restyled.io <commits@restyled.io>

Co-authored-by: Jorge Orpinel <jorgeorpinel@users.noreply.github.com>
Co-authored-by: restyled-io[bot] <32688539+restyled-io[bot]@users.noreply.github.com>
Co-authored-by: Restyled.io <commits@restyled.io>

content/docs/command-reference/params/diff.md

@@ -7,7 +7,8 @@ commits in the <abbr>DVC repository</abbr>, or between a commit and the
 ## Synopsis
 
 ```usage
-usage: dvc params diff [-h] [-q | -v] [--all] [--show-json] [--show-md]
+usage: dvc params diff [-h] [-q | -v] [--targets [<path> [<path> ...]]]
+                       [--all] [--show-json] [--show-md] [--no-path]
                        [a_rev] [b_rev]
 
 positional arguments:
@@ -35,6 +36,20 @@ itself does not ascribe any specific meaning for these values.
 
 ## Options
 
+- `--targets <paths>` - limit command scope to these params files. When
+  specifying arguments for `--targets` before `revisions`, you should use `--`
+  after this option's arguments, e.g.:
+
+  ```dvc
+  $ dvc params diff --targets m1.json m2.yaml -- HEAD v1
+  ```
+
+  Alternatively, you can also run the above statement as:
+
+  ```dvc
+  $ dvc params diff HEAD v1 --targets m1.json m2.json
+  ```
+
 - `--all` - prints all parameters including not changed.
 
 - `--show-json` - prints the command's output in easily parsable JSON format,