if: actually use if to test a command's success (#5344)

Using `{{command}} && {{command1}} || {{command2}}` does not actually
work like if, and it's misleading to teach that to users. The difference
is that in the above construct, command2 gets executed if command1
fails, which does not happen in a real if as in `if {{command}}; then
{{command1}}; else {{command2}}; fi`

pages/common/if.md

@@ -4,7 +4,7 @@
 
 - Echo a different thing depending on a command's success:
 
-`{{command}} && echo "success" || echo "failure"`
+`if {{command}}; then echo "success"; else echo "failure"; fi`
 
 - Full if syntax: