docs: add clarifying explanation about reference documentation (#442442)

doc/README.md

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+### Writing Function Documentation
+
+Function documentation is *reference documentation*, for which
+[diataxis Reference documentation](https://diataxis.fr/reference/) (8 minutes) is **mandatory reading**.
+
+On top of the diataxis framework, which provides a balanced perspective on what reference documentation should contain, we apply a specific style rule to function documentation:
+the first sentence is in present tense, active voice, and the subject is omitted, referring implicitly to the name of the function.
+For example:
+
+```nix
+/**
+  Subtracts value `b` from value `a`.
+
+  Returns the difference as a number.
+*/
+subtractValues # ...elided code
+```
+
+Renders as:
+
+```md
+## `subtractValues`
+
+Subtracts value `b` from value `a`.
+
+Returns the difference as a number.
+```
+
 ### Callouts and examples
 
 Use the [admonition syntax](#admonitions) for callouts and examples.