Document merge options more fully

The merge options in the taxonomy edit form are not (IMO) clearly documented.

The checkbox options provided are,

* Collect all parents of selected terms an add it to the merged term
* Collect all children of selected terms an add it to the merged term
* Collect all relations of selected terms an add it to the merged term

The README.txt says this (but doesn't reference those options) -

 - Merging
     With the merging action, you can put terms with the same meaning together (e.g. your vocabulary
     contains: SoC, Summer of Code, GSoC, Google Summer of Code). All terms, that get merged into 
     one other, get synonyms of resulting term (here called merged or main term). Additional
     all term-node association gets automatically updated (this means nodes, that had a merging term
     assigned, now get the resulting merged term instead). All merging terms are deleted afterwards. 
     In the Taxonomy Manager, you can do that by selecting all terms you want to merge and to click
     the 'Merge' button. A fieldset with an autocomplete field an some options expands. In the 
     autocomplete field you have to specify the resulting merged term (into which the selected get merged). 
     The merged term can be either chosen from the list of existing terms or can be inserted automatically
     and used as merged term.
     Additional, there are some options available (they depend on the vocabulary settings). If you want
     to add any kind of relations (parents, children, related terms) from the merging terms to the
     resulting merged term, select one (or more) of them.
     
     The default taxonomy term page, which shows all assigned nodes, is overriden by the Taxonomy
     Manager, so that former merged terms can be considered (if someone calls a term, that was merged, 
     it redirects to the resulting merged term).
     
     NOTE: At the moment, the Taxonomy Manager only cares about the term-node association inserted
           into the term_node table (by the taxonomy module). If you are using any CCK modules, like 
           CCK Taxonomy or Content Taxonomy, which (can) save the term - node association in cck tables, 
           don't use the Merging action, because changes are not handled.
           If you are using Views filters instead of the default taxonomy term page, merged terms are 
           either respected.
           If you want to customize this by yourself or have some other module, you can use following 
           function taxonomy_manager_merge_get_main_term($tid) for getting the main term id (if there 
           is any main term, else return 0). The term merge history gets saved in the 
           taxonomy_manager_merge table (main_tid, merged_tid) and gets additional cached, so that 
           checking for a merged terms causes nearly no performance loss.