Document the alignment for refcounted allocations

We use the same definition as malloc().
13 jobs for rc-align in 20 minutes and 15 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #171015
cross-android_api21_arm64

00:00:55

passed #171016
cross-android_api28_arm64

00:01:34

passed #171017
cross-mingw64

00:01:17

passed #171308
fedora-autotools-x86_64

00:05:59

passed #171171
fedora-x86_64

00:03:40

passed #171172
freebsd-11
freebsd-11-x86_64

00:04:00

passed #171018
win32
msys2-mingw32

00:08:18

passed #171019
win32
vs2017-x64

00:07:48

failed #171170
fedora-autotools-x86_64

00:03:43

failed #171014
fedora-autotools-x86_64

00:04:38

failed #171013
fedora-x86_64

00:03:29

failed #171020
freebsd-11
freebsd-11-x86_64

00:03:49

 
  Coverage
passed #171021
coverage

00:01:53

75.3%