Clarify comments + add field handler to multipart sample

[ace-n]

Do not merge until the docs have been updated accordingly.

cc @marekbiskup

[codecov]

Codecov Report

Merging #609 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #609   +/-   ##
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  Coverage   48.52%   48.52%           
=======================================
  Files           1        1           
  Lines          68       68           
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  Hits           33       33           
  Misses         35       35

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[marekbiskup]

I'm a bit afraid that users may confuse events described above, with event that trigger a function. For instance, users may think that there are many function executions here.

I think the previous form ("This callback will be invoked") was a bit better

Also, busboy docs say that events are emitted, not triggered.

How about something like this:

• "This code is executed on..."
• "This code handles busboy events emitted on..."
• "This busboy events are emitted on..."

[marekbiskup]

I would add an empty line before:
const tmpdir = os.tmpdir();

Otherwise it looks like the comment "This object will accumulate all the uploaded files, keyed by their name" is for the two lines.

You can group this "const tmpdir" together with "const busboy"