feat[dev]: Clarify that mm options is (sometimes) ok (#7263)

sites/dev/docs/reference/api/part/config/options/mm/readme.mdx

@@ -5,8 +5,9 @@ title: Millimeter options
 :::warning
 
 While FreeSewing supports millimeter options, we recommend
-using [percentage options][1] and will not accept
-contributions that use millimeter options.
+using [percentage options][1]. While there are [good
+reasons](#when) to use millimeter options, we may reject
+contributions that (ab)use them outside of a valid use case.
 
 :::
 
@@ -32,7 +33,7 @@ options: {
 
 :::note COMMENT (by joost)
 
-##### What's wrong with millimeter options?
+#### What's wrong with millimeter options?
 
 Millimeter options do not scale.
 Parametric design is the _raison d'être_ of FreeSewing and that core belief
@@ -40,14 +41,21 @@ that things should seamlessly adapt goes out the window when you use a `mm`
 option because now you have a value that will not change based on the
 input measurements.
 
-You could argue that it's fine because _you can just lower the option_
-but that breaks the principle of _sensible defaults_ (aka no surprises).
-The fact that you can sidestep the bullet does not mean you're not creating
-a footgun.
-
 When you need a millimeter option, reach for a [snapped
 percentage option][1] instead.
 
+#### When to use millimeter options?
+
+There are some use-cases where a millimeter option makes sense.
+This is not an exhaustive list, so use your own judgement.
+
+###### When the option is a de-facto measurement {#when}
+
+When an option functions as a de-facto measurement, you should use a millimeter
+option. For example, when you are designing a tent pattern, the length of the
+tent makes perfect sense as a millimeter option as FreeSewing does not have a
+_length-of-tent_ measurements, and adding one does not make a lot of sense.
+
 :::
 
 [1]: /reference/api/part/config/options/pct