Rollup merge of #25087 - nham:improve_vec_docs, r=Gankro

 This commit does two things: it adds an example for indexing vectors, and it changes the \"Examples\" section to use full sentences.

This change was spurred by someone in the #rust IRC channel asking if there was a `.set()` method for changing the `i`-th value of a vector (they had missed that `Vec` implements `IndexMut`, which is easy to do if you're not aware of that trait).

diff --cc src/libcollections/vec.rs
Simple merge