help: improve text about automatically-distributed files

help: improve text about automatically-distributed files

This change fixes automake bug#7819.

* automake.in (usage): Distinguish between files that are always
automatically distributed when found, and those which are only
"under certain conditions".
* doc/automake.texi (Basics of Distribution): Update accordingly.
* tests/autodist-subdir.test: Update.
* tests/autodist-no-duplicate.test: Likewise.
* tests/autodist.test: Likewise.
(configure.in): Remove useless call to AM_MAINTAINER_MODE.