`ReadonlyMap` lacks documentation for `forEach`, `get`, `has` and `size`

Nobody has claimed this yet.

### ⚙ Compilation target

ES2015+

### ⚙ Library

lib.es2015.collection.d.ts

### Missing / Incorrect Definition

`ReadonlyMap` lacks documentation for `forEach`, `get`, `has` and `size`.

Hovering these members in an editor shows no description, unlike the corresponding members of `Map`. The documentation should ideally be copied from regular `Map` for consistency, exactly as was done for `ReadonlySet` in #63481 (fixed by #63483).

Note: the `ReadonlyMap` members declared in lib.es2015.iterable.d.ts (`entries`, `keys`, `values` and `[Symbol.iterator]`) are already documented; only the four members declared in lib.es2015.collection.d.ts lack documentation.

### Sample Code

```ts
declare const m: ReadonlyMap;

// Hover any of these: no documentation is shown
m.get("a");
m.has("a");
m.size;
m.forEach(v => console.log(v));
```

### Documentation Link

_No response_

Open lib.es2015.collection.d.ts and compare the documented members of Map with forEach, get, has and size on ReadonlyMap. Check the ReadonlySet documentation change from #63481/#63483 for consistency. Done means editor hovers show descriptions for all four ReadonlyMap members matching the corresponding Map documentation.

Written by the indexing model from the issue text.