pahole: Describe expected use of 'default' in the man page

pahole: Describe expected use of 'default' in the man page

the 'default' --btf_features option is useful for testing the latest set
of standard features used for BTF encoding, but will not be used in the
kernel Makefile.btf.  It functions as a useful shortcut for testing the
latest set of standard features.  Describe this in the manual page.

Suggested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Daniel Xu <dxu@dxuuu.xyz>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kate Carcia Poulin <kcarcia@redhat.com>
Link: https://lore.kernel.org/r/20240430095501.1959359-1-alan.maguire@oracle.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>