runtime: update description of GODEBUG=gctrace=1

For #44167.

Change-Id: I2dcd13cbe74e88de00e9fc51f9bd86e604a167df
Reviewed-on: https://go-review.googlesource.com/c/go/+/399300
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>
Run-TryBot: Emmanuel Odeke <[email protected]>
Auto-Submit: Emmanuel Odeke <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/runtime/extern.go

@@ -64,13 +64,15 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	Currently, it is:
 		gc # @#s #%: #+#+# ms clock, #+#/#/#+# ms cpu, #->#-># MB, # MB goal, # P
 	where the fields are as follows:
-		gc #        the GC number, incremented at each GC
-		@#s         time in seconds since program start
-		#%          percentage of time spent in GC since program start
-		#+...+#     wall-clock/CPU times for the phases of the GC
-		#->#-># MB  heap size at GC start, at GC end, and live heap
-		# MB goal   goal heap size
-		# P         number of processors used
+		gc #         the GC number, incremented at each GC
+		@#s          time in seconds since program start
+		#%           percentage of time spent in GC since program start
+		#+...+#      wall-clock/CPU times for the phases of the GC
+		#->#-># MB   heap size at GC start, at GC end, and live heap
+		# MB goal    goal heap size
+		# MB stacks  estimated scannable stack size
+		# MB globals scannable global size
+		# P          number of processors used
 	The phases are stop-the-world (STW) sweep termination, concurrent
 	mark and scan, and STW mark termination. The CPU times
 	for mark/scan are broken down in to assist time (GC performed in