Fix several rustdoc warnings

[FreezyLemon]

See cargo doc for the warnings. Just some trivial things, like URLs broken across multiple lines and empty code blocks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.54%. Comparing base (f7689fc) to head (b3a4381).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
- Coverage   93.54%   93.54%   -0.01%     
==========================================
  Files          53       53              
  Lines       11662    11657       -5     
==========================================
- Hits        10909    10904       -5     
  Misses        753      753              

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[YeungOnion]

References can italicize with markdown underscore instead of HTML considering we use backticks for code.

All else is good, thanks again!