Here is a possible update for the documentation:

1562,1563c1562,1566
<  *  Adds the contents of <i>other_hash</i> to <i>hsh</i>, overwriting
<  *  entries with duplicate keys with those from <i>other_hash</i>.
---
 >  *  Adds the contents of <i>other_hash</i> to <i>hsh</i>.  If no
 >  *  block is specified entries with duplicate keys are overwritten
 >  *  with the values from <i>other_hash</i>, otherwise the value
 >  *  of each duplicate key is detemined by calling the block with
 >  *  the key, its value in <i>hsh</i> and its value in 
<i>other_hash</i>.
1567a1571,1576
 >  *
 >  *     h1 = { "a" => 100, "b" => 200 }
 >  *     h2 = { "b" => 254, "c" => 300 }
 >  *     h1.merge!(h2) { |key, v1, v2| v1 }
 >  *                     #=> {"a"=>100, "b"=>200, "c"=>300}
 >  *

-Charlie