vec![elem; size] with run-time size should be highly visible in docs for std::vec, std::vec::Vec and std::vec!

[petrochenkov]

It's a common beginner's problem to create a vector with run-time size filled with identical values, but the idiomatic solution is pretty much undiscoverable without asking people on IRC/StackOverflow/Reddit.

cc @steveklabnik

[steveklabnik]

It's already covered in the book at least, and I thought in some of these places too. Duplicating thing everywhere isn't the answer, though. If it's not on the macro or std::vec docs, it should be, though.

[steveklabnik]

This usage is already mentioned

1. in the macro docs http://doc.rust-lang.org/stable/std/macro.vec!.html
2. in the std::vec docs http://doc.rust-lang.org/stable/std/vec/index.html

[petrochenkov]

@steveklabnik
Duplication is necessary in this case because the syntax is "exotic" and not easily discoverable, it's not a normal method which everyone knows where to find.
Also, size with runtime value has to be actually used in an example and not only mentioned in text (that's what I meant by "highly visible").

The book doesn't count because nobody starts searching in the book when met with a question "How do I create a vector of N elements?", people try to look up it in the reference (in the method list of Vec, primarily).