Add a syntax check, so this doesn't recur. Reorganize existing checks.

Add a syntax check, so this doesn't recur.  Reorganize existing checks.

* doc/Makefile.am (sc-exponent-grouping):
(syntax_checks): New variable.
(sc-avoid-io, sc-avoid-non-zero, sc-avoid-timezone):
(sc-avoid-zeroes, sc-use-small-caps-NUL): New rules, extracted
from check-texinfo.
(check-texinfo): Depend on $(syntax_checks).