The `message-syntax-checks' option doesn't have a very useful
customization :type, and the manual section explaining what it does
isn't in the most obvious spot. This patch moves the manual explanation
from "News Headers" to the more general "Message Headers" section, as
the option governs both mail and news. It also expands the :type
definition to explicitly list all the possible options.

The defcustom originally had two comments on it, one saying "Guess this
one shouldn't be easy to customize...", and another on the :type line
saying "improve this". That seems a bit contradictory.

Anyway, I don't see the point of actually obfuscating the use of this
option; this patch should make it a little more discoverable.

What would be even better is to obsolete this in favor of a new
`message-disabled-syntax-checks' option that was simply a plain list of
symbols.

Eric