docs: report few more automake parsing limitations Partly motivated by automake bug#8360. * doc/automake.texi (General Operation): Report few more automake limitations w.r.t. parsing of unusual makefile constructs. Related minor reorderings. * tests/doc-parsing-buglets-colneq-subst.test: New test. * tests/doc-parsing-buglets-tabs.test: Likewise. * tests/Makefile.am (TESTS): Update.