document all remaining unsafe usages

[robjtede]

PR Type

Safety / Docs

PR Checklist

Check your PR fulfills the following:

• Documentation comments have been added / updated.
• Format code with the latest stable rustfmt

Overview

Rounds off our safety drive for v3 by documenting all remaining non-trivial usages of unsafe blocks and functions. Trivial cases include standard prepare_uninitialized_buffer methods and were not considered. One or two debug assertion were added.

[cdbattags]

@robjtede this is amazing work, sir

[codecov-commenter]

Codecov Report

Merging #1642 into master will increase coverage by 0.00%.
The diff coverage is 60.00%.

@@           Coverage Diff           @@
##           master    #1642   +/-   ##
=======================================
  Coverage   53.17%   53.18%           
=======================================
  Files         126      126           
  Lines       11836    11836           
=======================================
+ Hits         6294     6295    +1     
+ Misses       5542     5541    -1     

Impacted Files	Coverage Δ
actix-http/src/config.rs	58.71% <ø> (ø)
actix-http/src/h1/decoder.rs	61.85% <ø> (ø)
actix-http/src/h1/dispatcher.rs	55.64% <ø> (ø)
actix-http/src/h2/dispatcher.rs	0.00% <0.00%> (ø)
actix-http/src/macros.rs	0.00% <ø> (ø)
actix-http/src/ws/mask.rs	0.00% <ø> (ø)
actix-http/src/h1/encoder.rs	63.30% <85.71%> (+0.65%)	⬆️

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[Neopallium]

Looks good.