[MERGE] improve documentation of send/merge relationship

[Aaron Bentley]

Aaron Bentley has voted resubmit.
Status is now: Resubmit
Comment:
It's a bit of a half-truth to say that "Merge directives are applied 
using the merge command.", because they can also be used with the pull 
command, and will probably be supported by the branch command in the 
future.  And I agree that BRANCH_OR_MERGE_DIRECTIVE is too long. 
LOCATION is better.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C20080316191706.GA52058%40hyperion.scode.org%3E