docs: document AM_MISSING_PROG

docs: document AM_MISSING_PROG

Addresses automake bug#11793.

This macro has been present for a long time, and coreutils has been
relying on it despite no documentation, which argues that it is stable
enough to be worth documenting.

Furthermore, since we are hoping to change our preferred invocation
from 'missing --run program' to 'missing program' in a future version
of automake, we need a way for packages to consistently get the
preferred invocation form rather than open-coding a call to 'missing'.
In particular, the Autoconf manual would love to recommend this macro
when discussing how to integrate an autotest suite with automake.

* doc/automake.texi (Public Macros): Document AM_MISSING_PROG.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>