Improve documentation

I had [a misunderstanding](https://github.com/JuliaMath/FFTW.jl/pull/60#issuecomment-373695989) of `plan_*` functions.

Main reason of this was from the term "inverse" on [documentation](https://juliamath.github.io/AbstractFFTs.jl/stable/api.html#AbstractFFTs.plan_fft)

It says
> You can compute the inverse-transform plan by inv(P) and apply the inverse plan with P \ Â (the inverse plan is cached and reused for subsequent calls to inv or \), and apply the inverse plan to a pre-allocated output array A with ldiv!(A, P, Â).

The purpose of this explanation was
* you have FORWARD transform plan
* using `mul!` : FORWARD transform
* using `ldiv!` : BACKWARD transform

`plan_fft` is for FORWARD transform so it doesn't matter.

However, other `plan_*` documentation says basically "Same as `plan_fft`", so if you apply this structure to BACKWARD transform, documentation have two meaning
* you have BACKWARD transform plan
* using `mul!` : BACKWARD transform
* using `ldiv!` : FORWARD transform
or
* you have BACKWARD transform plan
* using `ldiv` : BACKWARD transform

because the term "INVERSE" and "BACKWARD" are often used interchangeably. It was hard to distinguish without investigating source code. I know there is a "-" between inverse and transform, but it is really confusing.

Would we have more clear explanation of this?