perf config: Document the PERF_CONFIG environment variable

There was a provision for setting this variable, but not the
getenv("PERF_CONFIG") call to set it, as this was fixed in the previous
cset, document that it can be used to ask for using an alternative
.perfconfig file or to disable reading whatever file exists in the
system or home directory, i.e. using:

  export PERF_CONFIG=/dev/null

Cc: Adrian Hunter <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Luis Cláudio Gonçalves <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Taeung Song <[email protected]>
Link: https://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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