docs(extmark): remove message about passing in id=0

The docs for `nvim_buf_set_extmark` mention that you can create a new
extmark when passing in `id=0`, however if you do this you'll get an
error since the code checks that id is positive.

```
id is not a positive integer
```

This change re-words the sentence to make it clearer.

src/nvim/api/extmark.c

@@ -366,12 +366,11 @@ Array nvim_buf_get_extmarks(Buffer buffer, Integer ns_id, Object start, Object e
 
 /// Creates or updates an extmark.
 ///
-/// To create a new extmark, pass id=0. The extmark id will be returned.
-/// To move an existing mark, pass its id.
-///
-/// It is also allowed to create a new mark by passing in a previously unused
-/// id, but the caller must then keep track of existing and unused ids itself.
-/// (Useful over RPC, to avoid waiting for the return value.)
+/// By default a new extmark is created when no id is passed in, but it is also
+/// possible to create a new mark by passing in a previously unused id or move
+/// an existing mark by passing in its id. The caller must then keep track of
+/// existing and unused ids itself. (Useful over RPC, to avoid waiting for the
+/// return value.)
 ///
 /// Using the optional arguments, it is possible to use this to highlight
 /// a range of text, and also to associate virtual text to the mark.