bug#46039: 26.3; Mention how to get _only_ syntactic font lock

[Drew Adams]

Please see this question (and comments):

https://emacs.stackexchange.com/q/62926/105

I supposed, but wasn't sure, that the answer is to just set
`font-lock-keywords' to nil (e.g. using `font-lock-defaults').

I looked for doc that spoke to this directly, but didn't find any.  The
doc tells you how to do the _opposite_: get only search-based
font-locking, and not also syntactic font-locking.  But I didn't find
anything explicitly saying how to get only syntactic font-locking.

If I'm right that this isn't mentioned explicitly, please consider
mentioning it.  It's a reasonable use case.



In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.18362
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''