Add error documents

crates/py2erg/error.rs

@@ -5,7 +5,6 @@ use erg_compiler::error::CompileError;
 
 pub(crate) fn reassign_func_error(
     input: Input,
-    errno: usize,
     loc: Location,
     caused_by: String,
     name: &str,
@@ -19,7 +18,7 @@ pub(crate) fn reassign_func_error(
                 "traditional_chinese" => format!("{name}已宣告，已被引用。不建議再次賦值"),
                 "english" => format!("{name} has already been declared and referenced. It is not recommended to reassign such a function"),
             ),
-            errno,
+            1,
             ErrorKind::AssignError,
             loc,
         ),

docs/source/errors.md

@@ -0,0 +1,12 @@
+# Pylyzer-specific errors
+
+## E0001: Reassignment of a function referenced by other functions
+
+```python
+def g(): return f()
+
+def f(): return 1
+def f(): return "a" # E0001: Reassignment of a function referenced by other functions
+
+print(g())
+```