docs(ref): Add --workspace as a key point

This is a frequent reason I use them

Author: epage

src/doc/src/reference/workspaces.md

@@ -5,6 +5,7 @@ members*, that are managed together.
 
 The key points of workspaces are:
 
+* Common commands can run across all workspace members, like `cargo check --workspace`.
 * All packages share a common `Cargo.lock` file which resides in the
   *workspace root*.
 * All packages share a common [output directory], which defaults to a