docs: update: add -R|--recursive

Fixes #1259

content/docs/command-reference/update.md

@@ -6,7 +6,7 @@ projects</abbr>, and corresponding [DVC-files](/doc/user-guide/dvc-file-format).
 ## Synopsis
 
 ```usage
-usage: dvc update [-h] [-q | -v] [--rev <commit>] targets [targets ...]
+usage: dvc update [-h] [-q | -v] [--rev <commit>] [-R] targets [targets ...]
 
 positional arguments:
   targets        DVC-files to update.
@@ -44,6 +44,10 @@ dvc update --rev master
   > Note that this changes the `rev` field in the import stage, fixing it to the
   > revision.
 
+- `-R`, `--recursive` - determines the data artifacts to update by searching
+  each target directory and its subdirectories for DVC-files to inspect. If
+  there are no directories among the targets, this option is ignored.
+
 - `-h`, `--help` - prints the usage/help message, and exit.
 
 - `-q`, `--quiet` - do not write anything to standard output. Exit with 0 if no