Add a note on relocatable pkg-config files to reference manual.

docs/markdown/Pkgconfig-module.md

@@ -125,3 +125,9 @@ different behaviour.
 - Once a pkg-config file is generated for a library using `pkg.generate(mylib)`,
   any subsequent call to `pkg.generate()` where mylib appears, will generate a
   `Requires:` or `Requires.private` instead of a `Libs:` or `Libs.private:`.
+
+### Relocatable pkg-config files
+
+By default, the files generated by `pkg.generate` contain a hardcoded prefix path.
+In order to make them relocatable, a `pkgconfig.relocatable` builtin option is provided.
+See [Pkgconfig module options](Builtin-options.md#pkgconfig-module).