runtime: update gctrace docs and print lastStackScan instead of max

This change updates the gctrace docs to include stacks and globals in
the format line, and prints lastStackScan for "# MB stacks" instead of
maxStackScan, which is more accurate.

Fixes #54649.

Change-Id: Ibff2c390c9c9bf2b24b5b4e98ca346cc98d7cb2e
Reviewed-on: https://go-review.googlesource.com/c/go/+/425366
Run-TryBot: Michael Knyszek <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: mknyszek

src/runtime/extern.go

@@ -79,7 +79,7 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	error at each collection, summarizing the amount of memory collected and the
 	length of the pause. The format of this line is subject to change.
 	Currently, it is:
-		gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # P
+		gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # MB stacks, #MB globals, # P
 	where the fields are as follows:
 		gc #         the GC number, incremented at each GC
 		@#s          time in seconds since program start

src/runtime/mgc.go

@@ -1120,7 +1120,7 @@ func gcMarkTermination() {
 		print(" ms cpu, ",
 			work.heap0>>20, "->", work.heap1>>20, "->", work.heap2>>20, " MB, ",
 			gcController.lastHeapGoal>>20, " MB goal, ",
-			gcController.maxStackScan.Load()>>20, " MB stacks, ",
+			gcController.lastStackScan.Load()>>20, " MB stacks, ",
 			gcController.globalsScan.Load()>>20, " MB globals, ",
 			work.maxprocs, " P")
 		if work.userForced {