spec: use original terminology in metasyntax (cleanup)
The metasyntax used in the spec is exactly the Wirth Syntax
Notation (WSN), which eventually influenced EBNF. Add a link
but keep mentioning EBNF which is likely more commonly known.
Use the original terms in the productions. Specifically, use
the words "Term" and "Factor" rather than "Alternative" and
"Term".
The terminology cleanup also resolves an inconsistency in the
subsequent prose which is referring to the correct "terms" now.
While at it, add a production for the entire Syntax itself,
matching the original WSN definition.
Also, replace the two uses of "grammar" with "syntax" for
consistency ("syntax" is the prevalent term used throughout
the spec).
Fixes #50074.
Change-Id: If770d5f32f56f509f85893782c1dafbb0eb29b2e
Reviewed-on: https://go-review.googlesource.com/c/go/+/405814 Reviewed-by: Robert Findley <rfindley@google.com> Reviewed-by: Robert Griesemer <gri@google.com> Reviewed-by: Ian Lance Taylor <iant@golang.org>