ab: add new examples (#2514)

pages/common/ab.md

@@ -4,8 +4,16 @@
 
 - Execute 100 HTTP GET requests to given URL:
 
-`ab -n 100 {{url}}`
+`ab -n {{100}} {{url}}`
 
 - Execute 100 HTTP GET requests, processing up to 10 requests concurrently, to given URL:
 
-`ab -n 100 -c 10 {{url}}`
+`ab -n {{100}} -c {{10}} {{url}}`
+
+- Use keep alive:
+
+`ab -k {{url}}`
+
+- Set the maximum number of seconds to spend for benchmarking:
+
+`ab -t {{60}} {{url}}`