doc: update go1.3.txt for some performance changes

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/75350043

doc/go1.3.txt

@@ -21,6 +21,9 @@ os/exec: fix Command with relative paths (CL 59580044)
 regexp: add one-pass optimization from RE2 (CL 13345046)
 runtime/debug: add SetPanicOnFault (CL 66590044)
 runtime: output how long goroutines are blocked (CL 50420043)
+runtime: better handling of defers, reduces goroutine memory footprint by 2K (CL 42750044)
+runtime: faster GC: concurrent sweep, better parallelization, 8K pages (up to 50-70% pause reduction) (CL 46430043, 46860043, 58230043)
+runtime/race: faster by ~40% (CL 55100044)
 sync: add Pool (CL 41860043, 46010043)
 syscall: add Accept4 for freebsd (CL 68880043)
 syscall: add NewCallbackCDecl to use for windows callbacks (CL 36180044)