Add docstring for `AbstractCFDateTime`

[haakon-e]

It would be useful with a docstring for AbstractCFDateTime so that any mention of the symbol in the docs can be cross-referenced, for example: [`AbstractCFDateTime`](@ref).

Currently, the help page is auto-generated. Specifically, it would be useful with a user-friendly summary and explanations of the various subtypes of this abstract type.

help?> AbstractCFDateTime
search: AbstractCFDateTime AbstractMatrix AbstractUnitRange AbstractPattern

  No documentation found for public symbol.

  Summary
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  abstract type AbstractCFDateTime{T, Torigintuple}

  Subtypes
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  CFTime.DateTimeTAI{T, Torigintuple}
  CFTime.DateTimeUTC{T, Torigintuple}
  DateTime360Day{T, Torigintuple}
  DateTimeAllLeap{T, Torigintuple}
  DateTimeJulian{T, Torigintuple}
  DateTimeNoLeap{T, Torigintuple}
  DateTimeProlepticGregorian{T, Torigintuple}
  DateTimeStandard{T, Torigintuple}

  Supertype Hierarchy
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  AbstractCFDateTime{T, Torigintuple} <: TimeType <: Dates.AbstractTime <: Any

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ref: openjournals/joss-reviews#8487

[haakon-e]

If you're open to reconsidering names of various objects (ref: #47), I would suggest naming this object CFDateTime instead of AbstractCFDateTime. It's more concise, and all concrete implementations (DateTimeJulian, etc.) have names that are suffixed to DateTime anyways.

This follows precedence, for example with the abstract type Period from Dates.jl.

[Alexander-Barth]

I added a docstring to AbstractCFDateTime. I prefer the name starting with Abstract... for abstract types since it is quite common in julia, for example

AbstractArray       AbstractChannel     AbstractChar        AbstractDict
AbstractDisplay     AbstractFloat       AbstractIrrational  AbstractMatch
AbstractMatrix      AbstractPattern     AbstractRange       AbstractSet
AbstractSlices      AbstractString      AbstractUnitRange   AbstractVecOrMat

[Alexander-Barth]

I am closing the issue. Feel free to re-open if the issue persists.