Add missing headings and abstract to Summary of the Grammar (#82)

Fixes rdar://103095366

Author: amartini51

Sources/TSPL/TSPL.docc/ReferenceManual/SummaryOfTheGrammar.md

@@ -1,5 +1,7 @@
 # Summary of the Grammar
 
+Read the whole formal grammar.
+
 <!--
 
 === IMPORTANT ===
@@ -13,6 +15,8 @@ make the same change here also.
 
 -->
 
+## Lexical Structure
+
 > Grammar of whitespace:
 >
 > *whitespace* → *whitespace-item* *whitespace*_?_
@@ -383,6 +387,8 @@ make the same change here also.
 >
 > *postfix-operator* → *operator*
 
+## Types
+
 > Grammar of a type:
 >
 > *type* → *function-type*
@@ -493,6 +499,8 @@ make the same change here also.
 >
 > *type-inheritance-list* → *attributes*_?_ *type-identifier* | *attributes*_?_ *type-identifier* **`,`** *type-inheritance-list*
 
+## Expressions
+
 > Grammar of an expression:
 >
 > *expression* → *try-operator*_?_ *await-operator*_?_ *prefix-expression* *infix-expressions*_?_
@@ -797,6 +805,8 @@ make the same change here also.
 >
 > *optional-chaining-expression* → *postfix-expression* **`?`**
 
+## Statements
+
 > Grammar of a statement:
 >
 > *statement* → *expression* **`;`**_?_
@@ -1093,6 +1103,8 @@ make the same change here also.
 >
 > *platform-version* → *decimal-digits* **`.`** *decimal-digits* **`.`** *decimal-digits*
 
+## Declarations
+
 > Grammar of a declaration:
 >
 > *declaration* → *import-declaration*
@@ -1523,6 +1535,8 @@ make the same change here also.
 >
 > *actor-isolation-modifier* → **`nonisolated`**
 
+## Attributes
+
 > Grammar of an attribute:
 >
 > *attribute* → **`@`** *attribute-name* *attribute-argument-clause*_?_
@@ -1547,6 +1561,8 @@ make the same change here also.
 >
 > *balanced-token* → Any punctuation except  **`(`**,  **`)`**,  **`[`**,  **`]`**,  **`{`**, or  **`}`**
 
+## Patterns
+
 > Grammar of a pattern:
 >
 > *pattern* → *wildcard-pattern* *type-annotation*_?_
@@ -1605,6 +1621,8 @@ make the same change here also.
 >
 > *expression-pattern* → *expression*
 
+## Generic Parameters and Arguments
+
 > Grammar of a generic parameter clause:
 >
 > *generic-parameter-clause* → **`<`** *generic-parameter-list* **`>`**