Add links in docstrings

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

While reviewing the docs on https://docs.rs/arrow/5.0.0/arrow/array/trait.Array.html#implementors as part of #603 (thanks @novemberkilo !) I found a few structs that are not often used directly but that might not be clear from the docs

Are there any user-facing changes?

Add some doc links to the structs that users might be looking for

[codecov-commenter]

Codecov Report

Merging #605 (ea4ad49) into master (6c1a86e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #605   +/-   ##
=======================================
  Coverage   82.41%   82.41%           
=======================================
  Files         167      167           
  Lines       46240    46240           
=======================================
  Hits        38108    38108           
  Misses       8132     8132           

Impacted Files	Coverage Δ
arrow/src/array/array_binary.rs	92.23% <ø> (ø)
arrow/src/array/array_list.rs	95.51% <ø> (ø)
arrow/src/array/array_string.rs	97.85% <ø> (ø)

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