6728 enhance bundle config docs (#6939)

Fixes #6728



### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: Wenqi Li <[email protected]>

docs/source/config_syntax.md

@@ -88,6 +88,9 @@ where `#` indicates a sub-structure of this configuration file.
 
 _Description:_ `1` is referencing as an integer, which is used to index (zero-based indexing) the `preprocessing` sub-structure.
 
+Relative reference is supported by starting the reference with `#`. For example, `@#A` is to use `A` at the
+same config structure level, and `@##A` refers to `A` at one level above.
+
 ### To evaluate as Python expressions
 
 ```json
@@ -110,6 +113,12 @@ _Description:_ `$` followed by an import statement is handled slightly different
 Python expressions. The imported module `resnet18` will be available as a global variable
 to the other configuration sections. This is to simplify the use of external modules in the configuration.
 
+The config expressions may use `@` to reference other config items. For example, in `$lambda x: x + @a + @b`,
+`@a` and `@b` are references to other Python objects and are made available to the anonymous function
+as 'globals'.
+It's therefore possible to modify the Python objects within an expression, for example,
+`$lambda x: @my_list.pop() + x` will pop the last element from `@my_list` and add it to `x`.
+
 ### To textually replace configuration elements
 
 ```json
@@ -196,3 +205,4 @@ Details on the CLI argument parsing is provided in the
   the `<module>` if it is not a (optional) dependency of MONAI.
 - As "#" and "$" might be interpreted differently by the `shell` or `CLI` tools, may need to add escape characters
   or quotes for them in the command line, like: `"\$torch.device('cuda:1')"`, `"'train_part#trainer'"`.
+- For more details and examples, please see [the tutorials](https://github.com/Project-MONAI/tutorials/tree/main/bundle).