topics/vcs-support: clarify the egg fragment's syntax
This doesn't actually address the semantics of extras in the
egg fragment.
@@ -139,9 +139,16 @@ option.
pip looks at 2 fragments for VCS URLs:
- `egg`: For specifying the "project name" for use in pip's dependency
- resolution logic. eg: `egg=project_name`
+ resolution logic. e.g.: `egg=project_name`
+
+ The `egg` fragment may additionally contain an extras specifier, e.g.:
+ `egg=project_name[dev,test]`.
+
+ Both the project name and extras specifier must appear in the form
+ defined by [PEP 508](https://peps.python.org/pep-0508/).
+
- `subdirectory`: For specifying the path to the Python package, when it is not
- in the root of the VCS directory. eg: `pkg_dir`
+ in the root of the VCS directory. e.g.: `pkg_dir`
````{admonition} Example
If your repository layout is:
