docs: give a list of possible skip options

docs/configuration.md

@@ -799,6 +799,13 @@ pre-commit:
 
 You can skip all or specific commands and scripts using `skip` option. You can also skip when merging, rebasing, or being on a specific branch. Globs are available for branches.
 
+Possible skip values:
+- `rebase` - when in rebase git state
+- `merge` - when in merge git state
+- `merge-commit` - when current HEAD commit is the merge commit
+- `ref: main` - when on a `main` branch
+- `run: test ${SKIP_ME} -eq 1` - when `test ${SKIP_ME} -eq 1` is successful (return code is 0)
+
 **Example**
 
 Always skipping a command: