doc: document PCALIGN directive
Fixes #64283
Change-Id: Ie0167410fc5557143f0b2e17af56ad9fe34605cf
GitHub-Last-Rev:
77144c76d38ca261b6728ba12ff128d22553efa5
GitHub-Pull-Request: golang/go#64296
Reviewed-on: https://go-review.googlesource.com/c/go/+/544115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>