Each plugin in Eclipse typically has it's own corresponding help plugin. Sometimes, it is necessary to link help topic in one plugin from a help topic in another plugin. I was not sure how to do that. Found that we can do it by having the link in the following form:
<a href="PLUGINS_ROOT/com.mycompany.mypluginid/html/mytopic.html">My Topic</a>
Here, com.mycompany.mypluginid is the id of the target plugin where mytopic.html lives.
Note: Before Eclipse 3.2, the cross plugin help link used to be referenced like "../com.mycompany.mypluginid/html/mytopic.html". It is recommended not to use it anymore.