Fix for PR automake/346:

Fix for PR automake/346:
* automake.texi (Optional): Supress the documentation for
AC_PATH_XTRA, AC_FUNC_ALLOCA, AC_FUNC_ERROR_AT_LINE, AC_FUNC_FNMATCH,
AC_FUNC_GETLOADAVG, AC_FUNC_MEMCMP, AC_FUNC_MKTIME, AC_FUNC_OBSTACK,
AC_FUNC_STRTOD, AC_REPLACE_FUNCS, AC_REPLACE_GNU_GETOPT,
AC_STRUCT_ST_BLOCKS, AM_WITH_REGEX, AC_CHECK_TOOL, AC_CHECK_PROG,
AC_CHECK_PROGS, AC_PATH_PROG, and AC_PATH_PROGS.  Augment the
documentation of AC_LIBSOURCE and AC_SUBST to explain this
applies to any macro that calls them.