docs: why one fs driver over another

[efx]

I loved reading the mount documentation.

I found myself asking why should I care about one implementation over the other.

So here is that internal conversation!

[ghost]

@AkihiroSuda thanks for the insight into what the benefits actually are! Do you feel like it makes sense to have a section summarizing why you might want to choose one over the other?

If so I can replace the reasons I wrote with your explanation.