Fontify some lines the way `font-lock-fontify-buffer' would.
The lines could be a function or paragraph, or a specified number of lines.
If ARG is given, fontify that many lines before and after point, or 16 lines if
no ARG is given and `font-lock-mark-block-function' is nil.
If `font-lock-mark-block-function' non-nil and no ARG is given, it is used to
delimit the region to fontify.