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>

doc/go_spec.html

@@ -22,20 +22,23 @@ <h2 id="Introduction">Introduction</h2>
 </p>
 
 <p>
-The grammar is compact and simple to parse, allowing for easy analysis
+The syntax is compact and simple to parse, allowing for easy analysis
 by automatic tools such as integrated development environments.
 </p>
 
 <h2 id="Notation">Notation</h2>
 <p>
-The syntax is specified using Extended Backus-Naur Form (EBNF):
+The syntax is specified using a
+<a href="https://en.wikipedia.org/wiki/Wirth_syntax_notation">variant</a>
+of Extended Backus-Naur Form (EBNF):
 </p>
 
 <pre class="grammar">
+Syntax      = { Production } .
 Production  = production_name "=" [ Expression ] "." .
-Expression  = Alternative { "|" Alternative } .
-Alternative = Term { Term } .
-Term        = production_name | token [ "…" token ] | Group | Option | Repetition .
+Expression  = Term { "|" Term } .
+Term        = Factor { Factor } .
+Factor      = production_name | token [ "…" token ] | Group | Option | Repetition .
 Group       = "(" Expression ")" .
 Option      = "[" Expression "]" .
 Repetition  = "{" Expression "}" .
@@ -53,7 +56,7 @@ <h2 id="Notation">Notation</h2>
 </pre>
 
 <p>
-Lowercase production names are used to identify lexical tokens.
+Lowercase production names are used to identify lexical (terminal) tokens.
 Non-terminals are in CamelCase. Lexical tokens are enclosed in
 double quotes <code>""</code> or back quotes <code>``</code>.
 </p>
@@ -170,7 +173,7 @@ <h3 id="Tokens">Tokens</h3>
 <h3 id="Semicolons">Semicolons</h3>
 
 <p>
-The formal grammar uses semicolons <code>";"</code> as terminators in
+The formal syntax uses semicolons <code>";"</code> as terminators in
 a number of productions. Go programs may omit most of these semicolons
 using the following two rules:
 </p>
@@ -3038,7 +3041,7 @@ <h3 id="Composite_literals">Composite literals</h3>
 <p>
 The LiteralType's <a href="#Core_types">core type</a> <code>T</code>
 must be a struct, array, slice, or map type
-(the grammar enforces this constraint except when the type is given
+(the syntax enforces this constraint except when the type is given
 as a TypeName).
 The types of the elements and keys must be <a href="#Assignability">assignable</a>
 to the respective field, element, and key types of type <code>T</code>;