Between ffc5f1cc, when grammar.md was created by copying parts of the
reference, and 8cf25526, when all EBNF was removed from reference.md,
there were parts of the grammar that were updated in reference.md but
not grammar.md, and then they weren't copied over because they existed
already, but they were slightly out of date.

Example: the `path_item : ident | "self" ;` rule in Use declarations was
changed from "mod" to "self" in the reference in 195fd9a2 but wasn't
updated in the grammar.