DOCS: Make config setting clearer

[dr-thca]

I was looking for how to disable the lightbulb in the statusline here: https://nvimdev.github.io/lspsaga/lightbulb/

But I was unsure what key should be used in the config. I discovered that I could just do

        init = function() require('lspsaga').setup({
            lightbulb = {sign = false},
        }) end

but it would be nice that the docs showed that "lightbulb" is the key to use. It might be relevant for other docs as well.

Maybe change the title "Default options" to "Default options (lightbulb)" or show a code example

[glepnir]

make sense.

[mosheavni]

related #1359