perf: documentation for cycles_no_execute event

[Julia Lawall]

Hello,

Many of the x86 pipeline.json files have the brief description "Total
execution stalls" for both CYCLE_ACTIVITY.CYCLES_NO_EXECUTE and
CYCLE_ACTIVITY.STALLS_TOTAL.  Should the case for
CYCLE_ACTIVITY.CYCLES_NO_EXECUTE have a brief description that mentions
cycles?  Some of the files do have a public description that mentions
cycles, eg broadwellde/pipeline.json has "Counts number of cycles nothing
is executed on any execution port.", but others (eg ivytown/pipeline.json)
just have "Total execution stalls." twice.  Maybe "Cycles where nothing is
executed" would be ok for CYCLE_ACTIVITY.CYCLES_NO_EXECUTE?

thanks,
julia