Just: say that these variables are an interface, not an implementation detail

`set allow-duplicate-variables` lets a consuming root replace any of them, and
`just` cannot warn about an assignment that no longer overrides anything: renaming
one leaves the root parsing, listing and passing CI while silently falling back to
the value here, losing only the variable that moved.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: redsun82

misc/just/README.md

@@ -63,6 +63,19 @@ instead replaces it, and then both run, each over the files of the repository th
 defines it: bazel formatting asks bazel from the root of the checkout the files belong
 to, so that a repository formats its own files with its own pin.
 
+That last part is arranged by variables rather than by recipes. `set
+allow-duplicate-variables` in `defs.just` lets an importing justfile assign a variable
+defined here and have its value win, which is how a consuming root points the bazel
+formatter at its own workspace, its own buildifier and its own exclusions. The leading
+underscore says these are not meant to be run, not that they are private: any of them a
+root might reasonably want to redirect is an interface between the two repositories.
+
+Renaming one is therefore a breaking change that nothing reports. `just` has no notion
+of an assignment that fails to override, so a root assigning the old name keeps parsing,
+keeps listing, keeps passing CI, and silently reverts to the value here. Worse, a root
+overriding several loses only the renamed one, leaving a half-applied configuration.
+Rename freely, but say so when handing the change over.
+
 A directory that only makes sense when named explicitly can opt out of being found from
 above: