[MERGE] doc: send -o, and more on send in user guide

[Ian Clatworthy]

Ian Clatworthy has voted tweak.
Status is now: Conditionally approved
Comment:
As beuno indicated, the option text should say 'merge directive', not 
just directive.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C20080424012845.E35AE39002D%40grace.sourcefrog.net%3E