mail, search: update doc URLs (#81)

The docs added "standard" in their paths.
diff --git a/mail/mail.go b/mail/mail.go
index f7955aa..1ce1e87 100644
--- a/mail/mail.go
+++ b/mail/mail.go
@@ -51,7 +51,7 @@
 	Attachments []Attachment
 
 	// Extra mail headers.
-	// See https://cloud.google.com/appengine/docs/go/mail/
+	// See https://cloud.google.com/appengine/docs/standard/go/mail/
 	// for permissible headers.
 	Headers mail.Header
 }
diff --git a/search/doc.go b/search/doc.go
index da331ce..5208f18 100644
--- a/search/doc.go
+++ b/search/doc.go
@@ -79,7 +79,7 @@
 Search takes a string query to determine which documents to return. The query
 can be simple, such as a single word to match, or complex. The query
 language is described at
-https://cloud.google.com/appengine/docs/go/search/query_strings
+https://cloud.google.com/appengine/docs/standard/go/search/query_strings
 
 Search also takes an optional SearchOptions struct which gives much more
 control over how results are calculated and returned.
diff --git a/search/search.go b/search/search.go
index 1da2f2b..35a567d 100644
--- a/search/search.go
+++ b/search/search.go
@@ -504,7 +504,7 @@
 	Name string
 
 	// Expr is evaluated to provide a custom content snippet for each document.
-	// See https://cloud.google.com/appengine/docs/go/search/options for
+	// See https://cloud.google.com/appengine/docs/standard/go/search/options for
 	// the supported expression syntax.
 	Expr string
 }
@@ -651,7 +651,7 @@
 // SortExpression defines a single dimension for sorting a document.
 type SortExpression struct {
 	// Expr is evaluated to provide a sorting value for each document.
-	// See https://cloud.google.com/appengine/docs/go/search/options for
+	// See https://cloud.google.com/appengine/docs/standard/go/search/options for
 	// the supported expression syntax.
 	Expr string