Add some missing doc comments

As pointed out by the linter, some exported functions and types are
missing doc comments.
The linter warnings have been reduced from 194 to 116.
Not all missing comments have been added in this commit though.

transform/livereloadinject.go

@@ -18,6 +18,8 @@ import (
 	"fmt"
 )
 
+// LiveReloadInject returns a function that can be used
+// to inject a script tag for the livereload JavaScript in a HTML document.
 func LiveReloadInject(port int) func(ct contentTransformer) {
 	return func(ct contentTransformer) {
 		endBodyTag := "</body>"