Description
Fixes issue where instructions for renderProps in the react-slate documentation, if followed properly, produce an undesirable effect. This PR updates instructions such that a user can re-style the placeholder while still maintaining desirable default behavior

Issue
Fixes: #5782

Example

<Editable
  placeholder="Enter text here..."
  renderPlaceholder={({ attributes, children }) => {
    const styledAttributes = {
      ...attributes,
      style: {
        ...attributes.style,
        color: 'gray',
      },
    };
    return <div {...styledAttributes}>{children}</div>;
  }}
/>

Context
It was noticed that, when following the docs exactly, for renderProps within react-slate that the placeholder was behaving strangely. It was then noticed that in the docs the style prop that came in from attributes was being completely overridden. This was creating a situation where the placeholder exhibited some strange and undesirable behavior. The documentation has been updated so that the attributes prop that comes in is extended upon instead of overwritten entirely. This allows a user to do things like change the color, opacity, etc without overwriting critical style related props.

Checks
No checks necessary; only a documentation change