runtime: update comment: modTimer is called by Timer.Reset
runtime: update comment: modTimer is called by Timer.Reset

Change-Id: I97d0d1343d41b603a68388e496411fb040dc6d66
GitHub-Last-Rev: d11177ad249bd844dd9e7e355eea28596d0b1fa8
GitHub-Pull-Request: golang/go#38625
Reviewed-on: https://go-review.googlesource.com/c/go/+/229767
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>

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