renice.md: Including common renice commands

pages/common/renice.md

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+# renice
+
+> Alters the scheduling priority/nicenesses of one or more running processes. Niceness values range from -20 (most favorable to the process) to 19 (least favorable to the process).
+
+- Change priority of a running process.
+
+`renice -n {{niceness_value}} -p {{pid}}`
+
+- Change priority of all processes owned by a user.
+
+`renice -n {{niceness_value}} -u {{user}}`
+
+- Change priority of all processes that belongs to a group.
+
+`renice -n {{niceness_value}} -g {{group}}`