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>

tools/perf/Documentation/perf-trace.txt

@@ -241,6 +241,11 @@ the thread executes on the designated CPUs. Default is to monitor all CPUs.
 	printing using the existing 'perf trace' syscall arg beautifiers to map integer
 	arguments to strings (pid to comm, syscall id to syscall name, etc).
 
+--force-btf::
+	Use btf_dump to pretty print syscall argument data, instead of using hand-crafted pretty
+	printers. This option is intended for testing BTF integration in perf trace. btf_dump-based
+	pretty-printing serves as a fallback to hand-crafted pretty printers, as the latter can
+	better pretty-print integer flags and struct pointers.
 
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