benchmark

Refine the User Guide CPU Frequency Scaling section (#1331)

* Revert "Refine docs on changing cpufreq governor (#1325)"

This reverts commit 9e859f5bf55073a2b4c785b97b9cb78cbdaf127e.

* Refine the User Guide CPU Frequency Scaling section

The text now describes the cpupower command, so users in a hurry
have something to copy/paste that will likely work.  It then
suggests that there are probably more convenient optons available
that people can look into.

This reverts the prior commit, which introduced a shell script
that doesn't work.  It also retains the spirit of the original
fix: no longer recommend setting the frequency governor to
"powersave", which might not be appropriate or available.

Note: I did attempt to write a bash script that set the govenor
to "powersave" for the duration of a single command, but I gave
up for many reasons:

 1) it got complex, in part because the cpupower command does not
 seem to be designed for scripts (e.g. it prints out complex
 English phrases).

 2) munging /proc/sys files directly feels unstable and less than
 universal.  The libcpupower and cpupower are designed to abstract
 those away, because the details can vary.

 3) there are better options.  E.g. various GUI programs, and
 even Gnome's core Settings UI, let you adjust the system's
 performance mode without root access.

Fixes #1325, #1327