Add Exceptions section to Boot starter ref docs

See gh-121

spring-graphql-docs/src/docs/asciidoc/boot-starter.adoc

@@ -173,6 +173,19 @@ values of interest in <<index.adoc#execution-context-webmvc,Spring MVC>>.
 
 
 
+[[boot-graphql-exception-handling]]
+== Exception
+
+Spring GraphQL enables applications to register one or more Spring
+`DataFetcherExceptionResolver` components that are invoked sequentially until one
+resolves the Exception to a list of `graphql.GraphQLError` objects. See
+<<execution-exceptions>> for details.
+
+The Boot starter detects beans of type `DataFetcherExceptionResolver` and registers them
+automatically with the `GraphQlSource.Builder`.
+
+
+
 [[boot-graphql-graphiql]]
 == GraphiQL
 
@@ -187,7 +200,6 @@ spring.graphql.graphiql.path=/graphiql
 
 
 
-
 [[boot-graphql-metrics]]
 == Metrics