diff --git a/doc/lispref/modes.texi b/doc/lispref/modes.texi
index 6d7c586e23c..668205d5c04 100644
--- a/doc/lispref/modes.texi
+++ b/doc/lispref/modes.texi
@@ -4118,16 +4118,17 @@ table in special cases.  @xref{Syntax Properties}.
 @defvar font-lock-keywords-only
 If the value of this variable is non-@code{nil}, Font Lock does not do
 syntactic fontification, only search-based fontification based on
-@code{font-lock-keywords}.  It is normally set by Font Lock mode based
-on the @var{keywords-only} element in @code{font-lock-defaults}.  If
-the value is @code{nil}, Font Lock will call @code{jit-lock-register}
-(@pxref{Other Font Lock Variables}) to set up for automatic
-refontification of buffer text following a modified line to reflect
-the new syntactic context due to the change.
+@code{font-lock-keywords}; this will usually have the effect of not
+fontifying comments and strings.  This variable is normally set by Font
+Lock mode based on the @var{keywords-only} element in
+@code{font-lock-defaults}.  If the value is @code{nil}, Font Lock will
+call @code{jit-lock-register} (@pxref{Other Font Lock Variables}) to set
+up for automatic refontification of buffer text following a modified
+line to reflect the new syntactic context due to the change.
 
-To use only syntactic fontification, this variable should
-be non-@code{nil}, while @code{font-lock-keywords} should be set to
-@code{nil} (@pxref{Font Lock Basics}).
+To use only syntactic fontification, both this variable and
+@code{font-lock-keywords} should be set to @code{nil} (@pxref{Font Lock
+Basics}).
 @end defvar
 
 @defvar font-lock-syntax-table
diff --git a/lisp/font-lock.el b/lisp/font-lock.el
index 0d1bd18ee23..0f3c303316b 100644
--- a/lisp/font-lock.el
+++ b/lisp/font-lock.el
@@ -560,8 +560,12 @@ This is normally set via `font-lock-add-keywords' and
 `font-lock-remove-keywords'.")
 
 (defvar font-lock-keywords-only nil
-  "Non-nil means Font Lock should not fontify comments or strings.
-This is normally set via `font-lock-defaults'.")
+  "Non-nil means Font Lock should not use syntactic fontifications.
+This is normally set via `font-lock-defaults'.  Setting it to a
+non-nil value will usually cause comments and strings not to be
+fontified, unless `font-lock-keywords' are set so as to fontify them,
+or some other font-lock mechanism has the same effect even when this
+variable is non-nil.")
 
 (defvar-local font-lock-keywords-case-fold-search nil
   "Non-nil means the patterns in `font-lock-keywords' are case-insensitive.