revised documentation of Cartesian Fourier operators

[philouc]

Fix documentation of Cartesian Fourier operators.

• Fix typos
• clarify dimensions
• Remove non-Cartesian features
• Homogeneize notation wrt dimensions, coils, etc.

[chaithyagr]

Changing to develop branch.

[codecov-commenter]

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@@            Coverage Diff             @@
##             develop     #147   +/-   ##
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  Coverage           ?   82.41%           
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  Files              ?       47           
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  Branches           ?        0           
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  Hits               ?     1889           
  Misses             ?      403           
  Partials           ?        0           

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unittests	82.41% <0.00%> (?)

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