1. It's a statutory right, so it doesn't need to be and generally isn't covered in shareholder agreements: By law in Delaware (where Dell was incorporated) and probably most other U.S. jurisdictions, minority shareholders who don't vote for a buy-out, and don't otherwise consent to it, can't stop the buy-out from going through, but they have the right to demand a judicial appraisal of the "true" value of their shares. [0] [1]
2. Also, public companies normally don't even have shareholder agreements among all shareholders. The articles of incorporation and the bylaws are probably the closest approximation. In many jurisdictions, the board of directors can unilaterally change the bylaws without shareholder approval. (Shareholder approval is often required for changes to the articles of incorporation, though.)
[0] https://www.sec.gov/Archives/edgar/data/878280/0001193125082...
[1] http://www.jonesday.com/newsknowledge/publicationdetail.aspx...