perf callchain: Update docs regarding kernel/user space unwinding

The method of unwinding for kernel space is defined by the kernel
config, not by the value of --call-graph.   Improve the documentation to
reflect this.

Signed-off-by: Tony Jones <tonyj@suse.de>
Link: http://lore.kernel.org/lkml/20200325164053.10177-1-tonyj@suse.de


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>