Document server-side pagination limit in README (#166)

The maximum effective page size for paginated tools is determined by the Gitea server's [`[api].MAX_RESPONSE_ITEMS`](https://github.com/go-gitea/gitea/blob/ffa626b585225d62718f39e1b5fcc00416b0b7e4/custom/conf/app.example.ini#L2471-L2472) setting (default: 50). Requesting a `perPage` value higher than this limit will be silently capped by the server.

Fixes https://gitea.com/gitea/gitea-mcp/issues/165

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/166
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

operation/milestone/milestone.go

@@ -33,7 +33,7 @@ var (
 		mcp.WithString("state", mcp.Description("milestone state (for 'list')"), mcp.DefaultString("all")),
 		mcp.WithString("name", mcp.Description("milestone name filter (for 'list')")),
 		mcp.WithNumber("page", mcp.Description("page number"), mcp.DefaultNumber(1)),
-		mcp.WithNumber("perPage", mcp.Description("results per page"), mcp.DefaultNumber(30)),
+		mcp.WithNumber("perPage", mcp.Description("results per page (may be capped by the server's MAX_RESPONSE_ITEMS setting, default 50)"), mcp.DefaultNumber(30)),
 	)
 
 	MilestoneWriteTool = mcp.NewTool(