Auto merge of #24924 - carols10cents:grammar-improvements, r=pnkfelix

I'm interested in helping out with #16676 but more in the grammar than the reference-- here's my first chunk, more to come!! :tada:

I did pull a bit *out* of the reference, though, that was more relevant to the grammar but wasn't moved over as part of #24729.

I'm looking at, e.g. https://github.com/rust-lang/rust/blob/master/src/libsyntax/ast.rs, as the source of truth, please let me know if I should be checking against something else instead/in addition.

r? @steveklabnik

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