feat(lsp): improve signature help display #35190

- Add delimiter between function signature and documentation, matching hover formatting
- Show title only if there are multiple clients or multiple signatures
- Avoid duplicating the title inside the window if it's already shown in the border

test/functional/plugin/lsp_spec.lua

@@ -3774,7 +3774,7 @@ describe('LSP', function()
         }
         return vim.lsp.util.convert_signature_help_to_markdown_lines(signature_help, 'cs', { ',' })
       end)
-      local expected = { '```cs', 'TestEntity.TestEntity()', '```', 'some doc' }
+      local expected = { '```cs', 'TestEntity.TestEntity()', '```', '---', 'some doc' }
       eq(expected, result)
     end)