doc/go1.12: add notes for syscall/js CLs 141644, 143137, 144384
doc/go1.12: add notes for syscall/js CLs 141644, 143137, 144384

Also update a Go 1 compatibility promise link to canonical URL.

Updates #27592
Updates #28264

Change-Id: I5994a0a63e0870c1795c65016590dfad829d26a7
Reviewed-on: https://go-review.googlesource.com/c/154618
Reviewed-by: Richard Musiol <eb533a4b7e679e5143b37a0022300174aaa46b21@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,josharian/go,CAFxX/go,akutz/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,cloudflare/go,josharian/go,golang/go,josharian/go,CAFxX/go,golang/go,akutz/go,nawawi/go,akutz/go,cloudflare/go,cloudflare/go,josharian/go,josharian/go,CAFxX/go,josharian/go,CAFxX/go,nawawi/go,akutz/go,nawawi/go,nawawi/go,golang/go,golang/go,cloudflare/go,golang/go,CAFxX/go,golang/go,akutz/go,golang/go,josharian/go,akutz/go,golang/go,josharian/go,cloudflare/go