Add a note about modifying TTL indexes to TTL page.

Consider these three documentation pages:
1. http://docs.mongodb.org/manual/tutorial/expire-data/
2. http://docs.mongodb.org/manual/tutorials/modify-an-index/
3. https://docs.mongodb.org/manual/reference/command/collMod/

I want to modify an existing TTL index to update the expireAfterSeconds.

I consult page 1, and learn about how to create a TTL index, and how they work. I consult 2, because I can use a search bar. Likely, I drop and create the TTL index as suggested there.

Later I consult 3, and learn that there is a better way.

Can we link or reference page 3 from page 1, to avoid such tragedy?