time: clarify Timer.Reset behavior on AfterFunc Timers
time: clarify Timer.Reset behavior on AfterFunc Timers

Fixes #28100

Change-Id: I37d4d7badf455e4ecf982d4fc7cb070052de2e45
Reviewed-on: https://go-review.googlesource.com/c/go/+/285632
Trust: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cloudflare/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,golang/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go