....[A] user may want to send a letter to many different people. If the user doesn't know about the mail merge feature, they will insanely copy and paste all the letters.
Having an index entry of mail merge is useless, because if the user doesn't about this feature, they can't look it up! However, having these index entries could help:
- distributing a letter to many recipients
- letters, sending a letter to many recipients
- mailing a letter to many recipients
- mass mailings, sending
- recipients, sending a letter to
- same letter, sending to many recipients
- sending a letter to many recipients
Yes, these are long index entries, but so what? A good index attempts to anticipate all the strange and wonderful ways a user might look up a topic.
The mail merge topic itself has to clearly explain why doing a mail merge is better than copying and pasting, because if the user cannot see the benefit of what you are suggesting, they won't do it.
"strange and wonderful" ... exactly :-)