Add explicit definition for BlockString (#1042)

Author: benjie

spec/Appendix B -- Grammar Summary.md

@@ -100,7 +100,7 @@ StringValue ::
 
 - `""` [lookahead != `"`]
 - `"` StringCharacter+ `"`
-- `"""` BlockStringCharacter\* `"""`
+- BlockString
 
 StringCharacter ::
 
@@ -121,6 +121,8 @@ HexDigit :: one of
 
 EscapedCharacter :: one of `"` `\` `/` `b` `f` `n` `r` `t`
 
+BlockString :: `"""` BlockStringCharacter\* `"""`
+
 BlockStringCharacter ::
 
 - SourceCharacter but not `"""` or `\"""`

spec/Section 2 -- Language.md

@@ -806,7 +806,7 @@ StringValue ::
 
 - `""` [lookahead != `"`]
 - `"` StringCharacter+ `"`
-- `"""` BlockStringCharacter\* `"""`
+- BlockString
 
 StringCharacter ::
 
@@ -827,6 +827,8 @@ HexDigit :: one of
 
 EscapedCharacter :: one of `"` `\` `/` `b` `f` `n` `r` `t`
 
+BlockString :: `"""` BlockStringCharacter\* `"""`
+
 BlockStringCharacter ::
 
 - SourceCharacter but not `"""` or `\"""`
@@ -1007,7 +1009,11 @@ StringCharacter :: `\` EscapedCharacter
 | {`r`}             | U+000D       | carriage return              |
 | {`t`}             | U+0009       | horizontal tab               |
 
-StringValue :: `"""` BlockStringCharacter\* `"""`
+StringValue :: BlockString
+
+- Return the _Unicode text_ by evaluating the {BlockString}.
+
+BlockString :: `"""` BlockStringCharacter\* `"""`
 
 - Let {rawValue} be the _Unicode text_ by concatenating the evaluation of all
   {BlockStringCharacter} (which may be an empty sequence).