…entation clearer

The Rustonomicon's Lifetimes chapter uses the idiom "big ask", which is obscure compared to "tall order" (check Google ngrams). Also, it's easily mistaken for a typo; either "a big task" or "a big thing to ask" could plausibly work there.

…abnik

…entation clearer

I could not use colors as suggested for rust-lang#29854 because Github doesn't support these in markdown, however this solution may be better for color-blind readers.

…ote, r=alexcrichton

r? @steveklabnik

The comma removes the sorface ambiguity

…r=steveklabnik

The Rustonomicon's Lifetimes chapter uses the idiom "big ask", which is obscure compared to "tall order" (check [Google ngrams](https://books.google.com/ngrams/graph?content=big+ask%2C+tall+order&year_start=1800)). Also, it's easily mistaken for a typo; either "a big task" or "a big thing to ask" could plausibly work there.

r? @steveklabnik