libperf cpumap: Document perf_cpu_map__nr()'s behavior

perf_cpu_map__nr()'s behavior around an empty CPU map is strange as it
returns that there is 1 CPU. Changing code that may rely on this
behavior is hard, we can at least document the behavior.

Reviewed-by: James Clark <[email protected]>
Signed-off-by: Ian Rogers <[email protected]>
Acked-by: Adrian Hunter <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Alexandre Ghiti <[email protected]>
Cc: Andrew Jones <[email protected]>
Cc: André Almeida <[email protected]>
Cc: Athira Rajeev <[email protected]>
Cc: Atish Patra <[email protected]>
Cc: Changbin Du <[email protected]>
Cc: Darren Hart <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Huacai Chen <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: John Garry <[email protected]>
Cc: K Prateek Nayak <[email protected]>
Cc: Kajol Jain <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mike Leach <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Paran Lee <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ravi Bangoria <[email protected]>
Cc: Sandipan Das <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Steinar H. Gunderson <[email protected]>
Cc: Suzuki Poulouse <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Yang Jihong <[email protected]>
Cc: Yang Li <[email protected]>
Cc: Yanteng Si <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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