If non-nil, means fontification should be syntactically true.
If nil, means fontification occurs only on those lines modified. This
means where modification on a line causes syntactic change on subsequent lines,
those subsequent lines are not refontified to reflect their new context.
If t, means fontification occurs on those lines modified and all
subsequent lines. This means those subsequent lines are refontified to reflect
their new syntactic context, after `jit-lock-context-time' seconds.
If any other value, e.g., `syntax-driven', means syntactically true
fontification occurs only if syntactic fontification is performed using the
buffer mode's syntax table, i.e., only if `font-lock-keywords-only' is nil.
The value of this variable is used when JIT Lock mode is turned on.