bg: clarify descriptions (#1416)

pages/common/bg.md

@@ -1,11 +1,11 @@
 # bg
 
-> Resumes suspended jobs and keeps them running in the background.
+> Resumes jobs that have been suspended (e.g. using `Ctrl + Z`), and keeps them running in the background.
 
-- Resume most recently suspended background job running in the background:
+- Resume most recently suspended job and run it in the background:
 
 `bg`
 
-- Resume a specific job running in the background:
+- Resume a specific job (use `jobs -l` to get its ID) and run it in the background:
 
 `bg {{job_id}}`