manifest-format: document implicit directory creation w/<copyfile> & <linkfile>
manifest-format: document implicit directory creation w/<copyfile> & <linkfile>

Bug: https://crbug.com/gerrit/11218
Change-Id: Ie96b4c484d9fbfd550c580c3d02971dc088dd8b0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/233052
Reviewed-by: David Pursehouse <e7b5c6a4a3dbaca972851f8c075e19915752593f@collab.net>
Tested-by: Jonathan Nieder <96e93edf89a1b75e35c5d08e01778cfc4fff73d9@google.com>

lipro-yocto/git-repo,codetutil/git-repo,lewixliu/git-repo,mer-tools/git-repo,codetutil/git-repo,aosp-mirror/tools_repo,GerritCodeReview/git-repo,couchbasedeps/git-repo,gabbayo/git-repo,lewixliu/git-repo,TheQtCompany/git-repo,mer-tools/git-repo,GerritCodeReview/git-repo,TheQtCompany/git-repo,couchbasedeps/git-repo,lipro-yocto/git-repo,aosp-mirror/tools_repo,gabbayo/git-repo