perf docs: Add description for stderr

perf docs: Add description for stderr

'perf report' displays recorded data on the screen and emits warnings
and debug messages in the status line (last one on screen).

perf also supports the possibility to write all debug messages to stderr
(instead of writing them to the status line).

This is achieved with the following command:

  # ./perf --debug stderr=1 report -vvvvv -i ~/fast.data 2>/tmp/2
  # ll /tmp/2
  -rw-rw-r-- 1 tmricht tmricht 5420835 May  7 13:46 /tmp/2
  #

The usage of variable stderr=1 is not documented, so add it to the perf
man page.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Link: http://lkml.kernel.org/r/20190513080220.91966-1-tmricht@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>