Add usage docs for combined_error macro

src/errors.rs

@@ -10,6 +10,20 @@ use std::sync::Arc;
 
 /// Combines errors into one error enum where `Error::source`, `Display` and `From<SourceError>` is already defined
 ///
+/// **Usage**
+/// ```no_run
+/// combined_error! {
+///     /// ANY DOCS HERE
+///     #[ANY_ATTRIBUTE]
+///     NAME_OF_ERROR_TYPE ";"
+///     ERROR_VARIANT "(" ERROR_TYPE ?STORED_TYPE ")" FORMAT_STRING ","
+/// }
+/// ```
+/// - Docs and attributes are optional and apply to the new error type
+/// - FORMAT_STRING: Format string passed directly to `write!` (the error is the only parameter passed to `write!`)
+/// - STORED_TYPE: Optional inner type (like Arc<ERROR_TYPE>)
+///   - where: ERROR_TYPE: Into<STORED_TYPE>
+///
 /// Additionally, all source errors are required to have a `Into<crate::Error>`
 /// implementation so that all errors can be converted to `crate::Error`
 macro_rules! combined_error {