perf docs: Add documentation for --force-btf option

The --force-btf option is intended for debugging purposes and is
currently undocumented. Add documentation for it.

Committer notes:

We need a follow up patch expanding on what can be done via BTF and what
isn't possible and thus needs further work to convert kernel C source
code into tables that can then be associated with syscall integer args
and struct members, as discussed in:

  https://lore.kernel.org/all/20241215190712.787847-3-howardchu95@gmail.com/T/#mcfbba653200775c59c730705229a49b34a153db7



Signed-off-by: Howard Chu <howardchu95@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20241215190712.787847-3-howardchu95@gmail.com
Link: https://lore.kernel.org/all/20241215190712.787847-3-howardchu95@gmail.com/T/#mcfbba653200775c59c730705229a49b34a153db7


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