crypto/rc4: remove false guarantees from Reset docs and deprecate it
crypto/rc4: remove false guarantees from Reset docs and deprecate it

Nothing in Go can truly guarantee a key will be gone from memory (see
#21865), so remove that claim. That makes Reset useless, because
unlike most Reset methods it doesn't restore the original value state,
so deprecate it.

Change-Id: I6bb0f7f94c7e6dd4c5ac19761bc8e5df1f9ec618
Reviewed-on: https://go-review.googlesource.com/c/162297
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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