docs: explain the purpose of order-only deps for generated files [skip ci]

The documentation of "order-only" dependencies is limited and their
various purposes are especially not clear. See issue #6391 for a recent
example, search the internet for many more. So mention the particular
purpose here while making the documentation barely longer.

docs/markdown/Reference-manual.md

@@ -547,11 +547,12 @@ libraries (`.so`, `.dll`, etc) will be linked.
 
 With the Ninja backend, Meson will create a build-time [order-only
 dependency](https://ninja-build.org/manual.html#ref_dependencies) on
-all generated input files, including unknown files. For all input
-files (generated and non-generated), Meson uses the [dependency
-file](https://ninja-build.org/manual.html#ref_headers) generated by
-your compiler to determine when to rebuild sources. The behavior is
-similar for other backends.
+all generated input files, including unknown files. This is needed
+to bootstrap the generation of the real dependencies in the
+[depfile](https://ninja-build.org/manual.html#ref_headers)
+generated by your compiler to determine when to rebuild sources.
+Ninja relies on this dependency file for all input files, generated
+and non-generated. The behavior is similar for other backends.
 
 Executable supports the following keyword arguments. Note that just
 like the positional arguments above, these keyword arguments can also