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

This makes reviews and suggestions much easier, since GitHub's review system is based on lines.

It also helps identifying long sentences at a glance.

### 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.