r2: make upload limit clearer (#26262)

Author: elithrar

src/content/docs/r2/platform/limits.mdx

@@ -14,7 +14,7 @@ import { Render } from "~/components";
 | Object key length                                                   | 1,024 bytes                  |
 | Object metadata size                                                | 8,192 bytes                  |
 | Object size                                                         | 5 TiB per object<sup>2</sup> |
-| Maximum upload size<sup>4</sup>                                     | 5 GiB<sup>3</sup>            |
+| Maximum upload size<sup>4</sup>                                     | 5 GiB (single-part) / 4.995TiB (multi-part) <sup>3</sup> |
 | Maximum upload parts                                                | 10,000                       |
 | Maximum concurrent writes to the same object name (key) | 1 per second <sup>5</sup> |