pulumi stack: add --all example (#14480)

pages/common/pulumi-stack.md

@@ -11,10 +11,14 @@
 
 `pulumi stack`
 
-- List known stacks:
+- List stacks in the current project:
 
 `pulumi stack ls`
 
+- List stacks across all projects:
+
+`pulumi stack ls --all`
+
 - Select an active stack:
 
 `pulumi stack select {{stack_name}}`