improve consistency in api code examples

[saadq]

Fix some minor inconsistencies in the docs:

• Use arrow functions throughout
• Consistent brace/semicolon/parens style
• Only accept next as a parameter if it needs to be called
• Use async functions throughout

Let me know if you'd like me to revert an of these.

[codecov]

Codecov Report

Merging #1029 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1029   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files           5        5           
  Lines         369      369           
=======================================
  Hits          368      368           
  Misses          1        1

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