is_from_for_loop: document what first check matches
is_from_for_loop: document what first check matches

Removing the first check will break a lot of for-loop UI tests and the
dogfood test.

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust