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 <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Taeung Song <treeze.taeung@gmail.com>
Link: https://lkml.kernel.org/n/tip-0u4o967hsk7j0o50zp9ctn89@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-config.txt

@@ -40,6 +40,10 @@ The '$HOME/.perfconfig' file is used to store a per-user configuration.
 The file '$(sysconfdir)/perfconfig' can be used to
 store a system-wide default configuration.
 
+One an disable reading config files by setting the PERF_CONFIG environment
+variable to /dev/null, or provide an alternate config file by setting that
+variable.
+
 When reading or writing, the values are read from the system and user
 configuration files by default, and options '--system' and '--user'
 can be used to tell the command to read from or write to only that location.