Clarify the use of `plan_fft!`

[JeffFessler]

The existing docstring refers to the array A that is used to construct the plan, but that array is irrelevant (other than its eltype and size) (Edit: and strides) when using the plan later. The old wording made it sound like the A in the constructor itself is used.

[codecov[bot]]

Codecov Report

Base: 84.13% // Head: 84.13% // No change to project coverage :thumbsup:

Coverage data is based on head (cf0f5af) compared to base (7d698db). Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #82   +/-   ##
=======================================
  Coverage   84.13%   84.13%           
=======================================
  Files           2        2           
  Lines         208      208           
=======================================
  Hits          175      175           
  Misses         33       33           

Impacted Files	Coverage Δ
src/definitions.jl	68.26% <ø> (ø)

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[JeffFessler]

bump :)