runtime: fix some out of date comments and typos

Change-Id: I061057414c722c5a0f03c709528afc8554114db6
Reviewed-on: https://go-review.googlesource.com/9367Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgcmark.go

@@ -660,8 +660,7 @@ func shade(b uintptr) {
 }
 
 // obj is the start of an object with mark mbits.
-// If it isn't already marked, mark it and enqueue into workbuf.
-// Return possibly new workbuf to use.
+// If it isn't already marked, mark it and enqueue into gcw.
 // base and off are for debugging only and could be removed.
 //go:nowritebarrier
 func greyobject(obj, base, off uintptr, hbits heapBits, span *mspan, gcw *gcWork) {

src/runtime/proc1.go

@@ -748,7 +748,7 @@ func mstart1() {
 // safe point and execute fn on that P. If the P is not executing code
 // (it is idle or in a syscall), this will call fn(p) directly while
 // preventing the P from exiting its state. This does not ensure that
-// fn will run on every CPU executing Go code, but it act as a global
+// fn will run on every CPU executing Go code, but it acts as a global
 // memory barrier. GC uses this as a "ragged barrier."
 //
 // The caller must hold worldsema.