adjuste doc of `map_while`
adjuste doc of `map_while`

Fix doc of `Iterator::map_while` so it would
be clearer that it isn't fused.

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