Move the EnumOrdinal.md doc to the right place (it got overwritten by automation).

Also, add a comment about manually providing an index if you need a stable one.

PiperOrigin-RevId: 609531242

Author: graememorgan

docs/bugpattern/EnumOrdinal.md

@@ -0,0 +1,38 @@
+You should almost never invoke the `Enum.ordinal()` method. The ordinal exists
+only to support low-level utilities like `EnumSet`. The ordinal of a given enum
+value is not guaranteed to be stable across builds because of the potential for
+enum values to be added, removed, or reordered.
+
+Prefer using enum value directly:
+
+```java
+ImmutableMap<MyEnum, String> MAPPING =
+    ImmutableMap.<MyEnum, String>builder()
+        .put(MyEnum.FOO, "Foo")
+        .put(MyEnum.BAR, "Bar");
+```
+
+to this:
+
+```java
+ ImmutableMap<Integer, String> MAPPING =
+    ImmutableMap.<MyEnum, String>builder()
+        .put(MyEnum.FOO.ordinal(), "Foo")
+        .put(MyEnum.BAR.ordinal(), "Bar");
+```
+
+Or if you need a stable number for serialisation, consider defining an explicit
+field on the enum instead:
+
+```java
+enum MyStableEnum {
+  FOO(1),
+  BAR(2),
+  ;
+
+  private final int index;
+  MyStableEnum(int index) {
+    this.index = index;
+  }
+}
+```