renice: split POSIX and util-linux implementations (#12047)

pages/linux/renice.md

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+# renice
+
+> Alter the scheduling priority/niceness of one or more running processes.
+> Niceness values range from -20 (most favorable to the process) to 19 (least favorable to the process).
+> See also: `nice`.
+> More information: <https://manned.org/renice>.
+
+- Set the absolute priority of a running [p]rocess:
+
+`renice {{+3}} -p {{pid}}`
+
+- Increase/decrease the priority of all processes owned by a [u]ser:
+
+`renice --relative {{-4}} -u {{uid|user}}`
+
+- Set the priority of all processes that belong to a process [g]roup:
+
+`renice --absolute {{5}} -g {{process_group}}`