lib.modules: document mkDefinition section

nixos/doc/manual/development/option-def.section.md

@@ -123,3 +123,47 @@ they were declared in separate modules. This can be done using
     ];
 }
 ```
+
+## Free-floating definitions {#sec-option-definitions-definitions}
+
+:::{.note}
+The module system internally transforms module syntax into definitions. This always happens internally.
+:::
+
+It is possible to create first class definitions which are not transformed _again_ into definitions by the module system.
+
+Usually the file location of a definition is implicit and equal to the file it came from.
+However, when manipulating definitions, it may be useful for them to be completely self-contained (or "free-floating").
+
+A free-floating definition is created with `mkDefinition { file = ...; value = ...; }`.
+
+Preserving the file location creates better error messages, for example when copying definitions from one option to another.
+
+Other properties like `mkOverride` `mkMerge` `mkAfter` can be used in the `value` attribute but not the other way around.
+
+The following shows an example configuration that yields an error with the custom position information:
+
+```nix
+{
+  _file = "file.nix";
+  options.foo = mkOption {
+    default = 13;
+  };
+  config.foo = lib.mkDefinition {
+    file = "custom place";
+    # mkOptionDefault creates a conflict with the option foo's `default = 1` on purpose
+    # So we see the error message below contains the conflicting values and different positions
+    value = lib.mkOptionDefault 42;
+  };
+}
+```
+
+evaluating the module yields the following error:
+
+```
+error: Cannot merge definitions of `foo'. Definition values:
+- In `file.nix': 13
+- In `custom place': 42
+```
+
+To set the file location for all definitions in a module, you may add the `_file` module syntax attribute, which has a similar effect to using `mkDefinition` on all definitions in the module, without the hassle.

nixos/doc/manual/redirects.json

@@ -1664,6 +1664,9 @@
   "sec-option-definitions-merging": [
     "index.html#sec-option-definitions-merging"
   ],
+  "sec-option-definitions-definitions": [
+    "index.html#sec-option-definitions-definitions"
+  ],
   "sec-assertions": [
     "index.html#sec-assertions"
   ],