git: openjdk/jdk11u-dev: 8247591: Document Alpine Linux build steps in OpenJDK build guide

[Alexander Scherbatiy]

Changeset: 19761f50
Author:    Aleksei Voitylov <avoitylov at openjdk.org>
Committer: Alexander Scherbatiy <alexsch at openjdk.org>
Date:      2022-03-30 12:50:47 +0000
URL:       https://git.openjdk.java.net/jdk11u-dev/commit/19761f50d2bbe0c4c8486dc4384a7d74f5c820fc

8247591: Document Alpine Linux build steps in OpenJDK build guide

Reviewed-by: phh
Backport-of: 508c8a95854a653bc85f8b931cf0d74521196e19

! doc/building.html
! doc/building.md