perf script: Update brstack syntax documentation

perf script: Update brstack syntax documentation

The following commits added new fields/flags to the branch stack field
list:

commit 1f48989cdc7d ("perf script: Output branch sample type")
commit 6ade6c646035 ("perf script: Show branch speculation info")
commit 1e66dcff7b9b ("perf script: Add not taken event for branch stack")

Update brstack syntax documentation to be consistent with the latest
branch stack field list. Improve the descriptions to help users
interpret the fields accurately.

Signed-off-by: Yujie Liu <yujie.liu@intel.com>
Reviewed-by: Leo Yan <leo.yan@arm.com>
Reviewed-by: Sandipan Das <sandipan.das@amd.com>
Link: https://lore.kernel.org/r/20250312072329.419020-1-yujie.liu@intel.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>