Clarify regex limits in tags API

We now know that these characters are indeed the only ones
supported in the search field, so let's note that info.

doc/api/tags.md

@@ -22,7 +22,7 @@ Parameters:
 | `id` | integer/string| yes | The ID or [URL-encoded path of the project](index.md#namespaced-path-encoding) owned by the authenticated user|
 | `order_by` | string | no | Return tags ordered by `name` or `updated` fields. Default is `updated` |
 | `sort` | string | no | Return tags sorted in `asc` or `desc` order. Default is `desc` |
-| `search` | string | no | Return list of tags matching the search criteria. You can use `^term` and `term$` to find tags that begin and end with `term` respectively. |
+| `search` | string | no | Return list of tags matching the search criteria. You can use `^term` and `term$` to find tags that begin and end with `term` respectively. No other regular expressions are supported. |
 
 ```json
 [