Add description to search box

[atsansone]

Hotlist: https://buganizer.corp.google.com/hotlists/5891206
Bug: https://buganizer.corp.google.com/issues/338006277

Observed Results

The "search" box does not have a clear description which explains its purpose.

• Screencast.

Expected Results

The search box should have a clear description, for example "search for articles in google".

User Impact

Users will get confused about what information they need to fill in the search field to find their required information. this will affect users who depend on screen readers, users such as who are low vision, blind.

Steps To Reproduce

1. Enable ChromeVox using Ctrl + Alt + z.
2. Go to https://dart.dev/search?cx=011220921317074318178%3A_yy-tmb5t_i&ie=UTF-8&hl=en&q=isolate.
3. Using the tab key navigate to "search" field.
4. Observe screen reader announcement.

Relevent GAR Sheet

A11Y Tracker.

GAR Details

1.11. Is the purpose of each link, control, and button clear?.

Environment Details

• Machine: Chromebook.
• Browser: Chrome (123.0.6312.112 (Official Build) (64-bit)).
• Accessibility Tool: ChromeVox.
• Bug Filling Time: 10 mins.