Rollup merge of #24981 - carols10cents:remove-more-priv, r=alexcrichton

Hi! While researching stuff for the reference and the grammar, I came across a few mentions of using the `priv` keyword that was removed in 0.11.0 (#13547, #8122, rust-lang/rfcs#26, [RFC 0026](https://github.com/rust-lang/rfcs/blob/master/text/0026-remove-priv.md)).

One occurrence is a mention in the reference, a few are in comments, and a few are marking test functions. I left the test that makes sure you can't name an ident `priv` since it's still a reserved keyword. I did a little grepping around for `priv `, priv in backticks, `Private` etc and I think the remaining instances are fine, but if anyone knows anywhere in particular I should check for any other lingering mentions of `priv`, please let me know and I would be happy to! :fallen_leaf: :ocean:

diff --cc src/doc/reference.md
Simple merge

diff --cc src/librustc_typeck/collect.rs
Simple merge

diff --cc src/libsyntax/parse/parser.rs
Simple merge