docs: Document when environment() was added [skip ci]
The environment() function was added in 0.35.0 (which contains
containing the actual addition)
@@ -456,7 +456,8 @@ Print the argument string and halts the build process.
environment_object environment()
```
-Returns an empty [environment variable object](#environment-object).
+Returns an empty [environment variable object](#environment-object). Added in
+0.35.0.
### executable()
