Update features.md

Author: jbteves-sandia

src/doc/src/reference/features.md

@@ -244,9 +244,11 @@ enabled:
   "foo bar"`). If building multiple packages in a [workspace], the
   `package-name/feature-name` syntax can be used to specify features for
   specific workspace members.
-
+* `--all-features`: Activates all features of all packages selected on the command line.
 * `--no-default-features`: Does not activate the [`default`
   feature](#the-default-feature) of the selected packages.
+  
+**NOTE**: check the individual subcommand documentation for details. Not all flags are available for all subcommands.
 
 [workspace]: workspaces.md