Two FAQ proposals - disable plugins and no file or directory

[RollingStar]

Two FAQ proposals:

How do I...

Temporarily disable plugins? (A: Not sure about this one. Setting plugins to '' (2 single quotes) seems to work.)

Why does beets…

Complain about "no such file or directory" (A: Run beet update with a query if you have removed files without using beets)

If these questions make sense for the FAQ I will write them up.

[sampsyo]

The second one certainly makes sense! But for the first one, I'm not sure how much to add other than "disable plugins the same way you enabled them"?

[RollingStar]

I guess commenting out the plugins line would work, since it should fall back to the default empty value.

https://github.com/beetbox/beets/blob/master/beets/config_default.yaml#L61

The reason I think this matters is disabling plugins is part of the bug report template.

[ts1a]

Related: #2232

[RollingStar]

#3749

For "Complain about "no such file or directory" ", I think it will be better to make the actual error more informative. See the bottom of #2486