Adds an entry for 'function' to the glossary (#5526)

Adds a new entry to the site glossary for the word "function." Text is
taken pretty much verbatim from a comment made by @munificent in #3602.
😄

I'm assigning this to @MaryaBelanger, since she literally watched me
make the change over GVC as I was testing the new 11ty infra.

Fixes #3602

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---------

Co-authored-by: Marya <111139605+MaryaBelanger@users.noreply.github.com>

src/content/resources/glossary.md

@@ -132,6 +132,15 @@ For additional details, see the
 
 [definiteAssignmentSpec]: https://github.com/dart-lang/language/blob/main/resources/type-system/flow-analysis.md
 
+## Function
+
+Unless noted otherwise, the term *function* refers to top-level functions, local functions, static
+methods, and instance methods.
+
+For additional details, see the documentation on [Functions][_functions_].
+
+[_functions_]: /language/functions
+
 ## Irrefutable pattern
 
 _Irrefutable patterns_ are patterns that always match.