perf script: Update brstack syntax documentation

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

commit 1f48989 ("perf script: Output branch sample type")
commit 6ade6c6 ("perf script: Show branch speculation info")
commit 1e66dcf ("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>

tools/perf/Documentation/perf-script.txt

@@ -239,13 +239,22 @@ OPTIONS
 	i.e., -F "" is not allowed.
 
 	The brstack output includes branch related information with raw addresses using the
-	/v/v/v/v/cycles syntax in the following order:
-	FROM: branch source instruction
-	TO  : branch target instruction
-        M/P/-: M=branch target mispredicted or branch direction was mispredicted, P=target predicted or direction predicted, -=not supported
-	X/- : X=branch inside a transactional region, -=not in transaction region or not supported
-	A/- : A=TSX abort entry, -=not aborted region or not supported
-	cycles
+	FROM/TO/EVENT/INTX/ABORT/CYCLES/TYPE/SPEC syntax in the following order:
+	FROM  : branch source instruction
+	TO    : branch target instruction
+	EVENT : M=branch target or direction was mispredicted
+	        P=branch target or direction was predicted
+	        N=branch not-taken
+	        -=no event or not supported
+	INTX  : X=branch inside a transactional region
+	        -=branch not in transaction region or not supported
+	ABORT : A=TSX abort entry
+	        -=not aborted region or not supported
+	CYCLES: the number of cycles that have elapsed since the last branch was recorded
+	TYPE  : branch type: COND/UNCOND/IND/CALL/IND_CALL/RET etc.
+	        -=not supported
+	SPEC  : branch speculation info: SPEC_WRONG_PATH/NON_SPEC_CORRECT_PATH/SPEC_CORRECT_PATH
+	        -=not supported
 
 	The brstacksym is identical to brstack, except that the FROM and TO addresses are printed in a symbolic form if possible.