Document safety justification of some uses of from_trusted_len_iter

[alamb]

Which issue does this PR close?

Re #197

Rationale for this change

As part of #197 I reviewed all uses of from_trusted_len_iter; While doing so I found some without a safety justification, and so I added the results of my review as comments

What changes are included in this PR?

Code comments justifying why (existing) calls to from_trusted_len_iter() are sound

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #1148 (2984b99) into master (719096b) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1148      +/-   ##
==========================================
- Coverage   82.55%   82.55%   -0.01%     
==========================================
  Files         169      169              
  Lines       50456    50456              
==========================================
- Hits        41655    41654       -1     
- Misses       8801     8802       +1     

Impacted Files	Coverage Δ
arrow/src/array/array_primitive.rs	94.66% <ø> (ø)
arrow/src/buffer/ops.rs	96.77% <ø> (ø)
arrow/src/compute/kernels/arithmetic.rs	84.14% <ø> (ø)
arrow/src/compute/kernels/length.rs	100.00% <ø> (ø)
parquet/src/encodings/encoding.rs	93.52% <0.00%> (-0.20%)	⬇️

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[jhorstmann]

LGTM 🚀