docs: extmarks indexing #15311

ref #11456

src/nvim/api/vim.c

@@ -1551,7 +1551,7 @@ void nvim_set_current_tabpage(Tabpage tabpage, Error *err)
   }
 }
 
-/// Creates a new namespace, or gets an existing one.
+/// Creates a new *namespace*, or gets an existing one.
 ///
 /// Namespaces are used for buffer highlights and virtual text, see
 /// |nvim_buf_add_highlight()| and |nvim_buf_set_extmark()|.