std.ArrayHashMap fix eql documentation

In cf88cf2 the eql function provided in
The context of ArrayHashMap was changed to also include the key index,
but this wasn't properly updated in the documentation.

Since a flat `usize` is unintuitive, I've tried to explain the function
of the parameter as best I can based on the original commit.

Finally, I didn't do an extensive search if this eql definition is
incorrectly stated anywhere outside of these 2 spots. But I somewhat
doubt an file outside of `array_hash_map` would

Author: NicoElbers

lib/std/array_hash_map.zig

@@ -80,8 +80,10 @@ pub fn ArrayHashMapWithAllocator(
     comptime V: type,
     /// A namespace that provides these two functions:
     /// * `pub fn hash(self, K) u32`
-    /// * `pub fn eql(self, K, K) bool`
+    /// * `pub fn eql(self, K, K, usize) bool`
     ///
+    /// The final `usize` in the `eql` function represents the index of the key
+    /// that's already inside the map.
     comptime Context: type,
     /// When `false`, this data structure is biased towards cheap `eql`
     /// functions and avoids storing each key's hash in the table. Setting
@@ -525,7 +527,10 @@ pub fn ArrayHashMapUnmanaged(
     comptime V: type,
     /// A namespace that provides these two functions:
     /// * `pub fn hash(self, K) u32`
-    /// * `pub fn eql(self, K, K) bool`
+    /// * `pub fn eql(self, K, K, usize) bool`
+    ///
+    /// The final `usize` in the `eql` function represents the index of the key
+    /// that's already inside the map.
     comptime Context: type,
     /// When `false`, this data structure is biased towards cheap `eql`
     /// functions and avoids storing each key's hash in the table. Setting