hook_load can overwrite information: should be documented

The description for hook_load says that It is called to allow the module a chance to load extra information that it stores about a node. and that the returned object will be merged with the passed-in $node to result in an object containing both sets of properties.

This is not exactly what happens: node.module/node_load does not exactly return the node containing both sets of properties, but the passed-in properties, plus the extra properties added by the hook_load implementation, which can replace the passed-in properties if the returned object contains keys having the same name as the passed-in properties, which means they are not only "extra" properties.

This means existing properties loaded in the first part of node_load can be overwritten, or even removed by hook_load (using unset), which should be
- either documented if this is the desired behaviour for the code,
- or avoided by sanitizing the return value of hook_load through checking for duplicates with the passed-in values.

Page affected:
http://drupaldocs.org/api/head/function/node_load
http://drupaldocs.org/api/4.6/function/node_load