Clarify that lexing is greedy

GraphQL syntactical grammars intend to be unambiguous. While lexical grammars should also be - there has historically been an assumption that lexical parsing is greedy. This is obvious for numbers and words, but less obvious for empty block strings.

Either way, the additional clarity removes ambiguity from the spec

Partial fix for #564

Specifically addresses #564 (comment)

spec/Appendix A -- Notation Conventions.md

@@ -48,10 +48,12 @@ ListOfLetterA :
 
 The GraphQL language is defined in a syntactic grammar where terminal symbols
 are tokens. Tokens are defined in a lexical grammar which matches patterns of
-source characters. The result of parsing a sequence of source Unicode characters
-produces a GraphQL AST.
+source characters. The result of parsing a source text sequence of Unicode
+characters first produces a sequence of lexical tokens according to the lexical
+grammar which then produces abstract syntax tree (AST) according to the
+syntactical grammar.
 
-A Lexical grammar production describes non-terminal "tokens" by
+A lexical grammar production describes non-terminal "tokens" by
 patterns of terminal Unicode characters. No "whitespace" or other ignored
 characters may appear between any terminal Unicode characters in the lexical
 grammar production. A lexical grammar production is distinguished by a two colon

spec/Section 2 -- Language.md

@@ -7,11 +7,13 @@ common unit of composition allowing for query reuse.
 
 A GraphQL document is defined as a syntactic grammar where terminal symbols are
 tokens (indivisible lexical units). These tokens are defined in a lexical
-grammar which matches patterns of source characters (defined by a
-double-colon `::`).
+grammar which matches patterns of source characters. In this document, syntactic
+grammar productions are distinguished with a colon `:` while lexical grammar
+productions are distinguished with a double-colon `::`.
 
-Note: See [Appendix A](#sec-Appendix-Notation-Conventions) for more details about the definition of lexical and syntactic grammar and other notational conventions
-used in this document.
+Note: See [Appendix A](#sec-Appendix-Notation-Conventions) for more information
+about the lexical and syntactic grammar and other notational conventions used
+throughout this document.
 
 
 ## Source Text
@@ -25,6 +27,19 @@ ASCII range so as to be as widely compatible with as many existing tools,
 languages, and serialization formats as possible and avoid display issues in
 text editors and source control.
 
+**Greedy Lexical Parsing**
+
+The source text of a GraphQL document is first converted into a sequence of
+lexical tokens, {Token}, and ignored tokens, {Ignored}. The source text is
+scanned from left to right, repeatedly taking the longest possible sequence of
+unicode characters as the next token.
+
+For example, the sequence `123` is always interpreted as a single {IntValue},
+and `""""""` is always interpreted as a single block {StringValue}.
+
+This sequence of lexical tokens are then scanned from left to right to produce
+an abstract syntax tree (AST) according to the {Document} syntactical grammar.
+
 
 ### Unicode
 
@@ -118,8 +133,7 @@ Token ::
 A GraphQL document is comprised of several kinds of indivisible lexical tokens
 defined here in a lexical grammar by patterns of source Unicode characters.
 
-Tokens are later used as terminal symbols in a GraphQL Document
-syntactic grammars.
+Tokens are later used as terminal symbols in GraphQL syntactic grammar rules.
 
 
 ### Ignored Tokens