doc: document new timer behavior

Change-Id: Ifa5894c67a36eb2c101b23b85775a3702512ca79
Reviewed-on: https://go-review.googlesource.com/c/go/+/571796
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

doc/next/6-stdlib/1-time.md

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+### Timer changes
+
+Go 1.23 makes two significant changes to the implementation of
+[`time.Timer`](/pkg/time#Timer) and [`time.Ticker`](/pkg/time#Ticker).
+
+First, `Timer`s and `Ticker`s that are no longer referred to by the program
+become eligible for garbage collection immediately, even if their
+`Stop` methods have not been called.
+Earlier versions of Go did not collect unstopped `Timer`s until after
+they had fired and never collected unstopped `Ticker`s.
+
+Second, the timer channel associated with a `Timer` or `Ticker` is
+now unbuffered, with capacity 0.
+The main effect of this change is that Go now guarantees
+that for any call to a `Reset` or `Stop` method, no stale values
+prepared before that call will be sent or received after the call.
+Earlier versions of Go used channels with a one-element buffer,
+making it difficult to use `Reset` and `Stop` correctly.
+A visible effect of this change is that `len` and `cap` of timer channels
+now returns 0 instead of 1, which may affect programs that
+poll the length to decide whether a receive on the timer channel
+will succeed.
+Such code should use a non-blocking receive instead.
+
+These new behaviors are only enabled when the main Go program
+is in a module with a `go.mod` `go` line using Go 1.23.0 or later.
+When Go 1.23 builds older programs, the old behaviors remain in effect.
+The new [GODEBUG setting](/doc/godebug) [`asynctimerchan=1`](/pkg/time/#NewTimer)
+can be used to revert back to asynchronous channel behaviors
+even when a program names Go 1.23.0 or later in its `go.mod` file.