Breaking change
We were previsouly using the description schema property to fill the placeholder attribute of text inputs; this is no more the case, you'll have to use the newly introduced ui:placeholder uiSchema directive instead.
Placeholders
Text fields can benefit from placeholders by using the ui:placeholder uiSchema directive:
const schema = {type: "string", format: "uri"};
const uiSchema = {
"ui:placeholder": "http://"
};