doc: go1.12: clarify use of MADV_FREE
doc: go1.12: clarify use of MADV_FREE

Fixes #29337

Change-Id: I1d632d19058c63dac8e25d2a5ad55555c1aec9d4
Reviewed-on: https://go-review.googlesource.com/c/155438
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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