Add explanation for "iife"

[DavidBruant]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Context

I was reading the rollup doc with a beginner in JavaScript
and i realized that the meaning of "iife" was not explained anywhere

Description

I'm adding the meaning in a place that i feel makes sense in the documentation
This is only my best guess

I'm happy to add the explanation elsewhere and write a different text if the one i proposed is not satisfying

[codecov]

Codecov Report

Merging #4020 (224260b) into master (319c370) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4020   +/-   ##
=======================================
  Coverage   97.44%   97.44%           
=======================================
  Files         192      192           
  Lines        6762     6762           
  Branches     1981     1981           
=======================================
  Hits         6589     6589           
  Misses         85       85           
  Partials       88       88           

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[lukastaegert]

Good addition!