Document the importer-without-url deprecation

Closes #1013

source/documentation/breaking-changes/importer-without-url.md

@@ -0,0 +1,39 @@
+---
+title: "Breaking Change: Importer without URL"
+introduction: |
+  The JS and Dart APIs used to allow passing an `importer` (singular) argument to
+  `compileString*()` without a corresponding `url`. This is now deprecated in
+  favor of passing the importer to `importers` (plural) instead.
+---
+
+The [`importer` option] to `compileString()` and `compileStringAsync()` (and the
+similar option in the Dart API) is intended to represent the importer that's
+used to resolve relative URLs in the entrypoint file. The `compile()` and
+`compileAsync()` functions don't have this option because they _always_ load the
+entrypoint from the filesystem, so they use the filesystem importer; but if
+you're loading the entrypoint from a Sass string you'll need to supply the
+importer yourself.
+
+[`importer` option]: /documentation/js-api/interfaces/stringoptionswithimporter/#importer
+
+Relative loads in Sass are handled by resolving the loaded URL (`path/to/style`
+in `@use "path/to/style"`) relative to the canonical URL for the file containing
+it; the `url` parameter provides that canonical URL. Logically, it doesn't make
+sense for an importer to handle relative loads without knowing what they're
+relative _to_. However, instead of throwing an error, Sass historically handled
+this by attempting to canonicalize the relative URL with the `importer`
+parameter the same way it would if it were in the `importers` array, by calling
+`canonicalize("path/to/style")`.
+
+This behavior was always wrong. The only importers whose `canonicalize()`
+functions should be passed relative URLs are the ones in the `importers`
+(plural) parameter. In most cases, if you're seeing this deprecation message,
+you can just move your importer to `importers` and it will continue to work.
+
+## Transition Period
+
+{% compatibility 'dart: "1.75.0"', 'libsass: false', 'ruby: false' %}{% endcompatibility %}
+
+Until Dart Sass 2.0.0, passing an `importer` without a `url` will produce a
+deprecation message named `importer-without-url` instead of throwing an error.
+

source/documentation/breaking-changes/index.md

@@ -22,6 +22,10 @@ time-sensitive, so they may be released with new minor version numbers instead.
 
 These breaking changes are coming soon or have recently been released:
 
+* [Passing `importer` without `url` to the JS and Dart `compileString*()` APIs
+  is deprecated](/documentation/breaking-changes/importer-without-url) beginning
+  in Dart Sass 1.75.0.
+
 * [Passing a percentage unit to the global `abs()` is
   deprecated](/documentation/breaking-changes/abs-percent/) beginning in Dart
   Sass 1.65.0.