doc: add a comment about the non_exhaustive feature

src/git.rs

@@ -21,6 +21,9 @@ use prepare::{GitWorkArea, WorkAreaResult};
 pub struct CommitId(String);
 
 /// Errors which may occur when working with workareas.
+///
+/// This enum is `non_exhaustive`, but cannot be marked as such until it is stable. In the
+/// meantime, there is a hidden variant.
 #[derive(Debug)]
 // TODO: #[non_exhaustive]
 pub enum GitError {

src/prepare.rs

@@ -42,6 +42,9 @@ pub enum SubmoduleIntent {
 }
 
 /// Errors which may occur when using a workarea.
+///
+/// This enum is `non_exhaustive`, but cannot be marked as such until it is stable. In the
+/// meantime, there is a hidden variant.
 #[derive(Debug)]
 // TODO: #[non_exhaustive]
 pub enum WorkAreaError {