store: Add optional 'references' parameter to 'text-file'.

* guix/store.scm (text-file): Add optional 'references' parameter.  Pass
  it to 'add-text-to-store'.
* doc/guix.texi (The Store Monad): Adjust accordingly.

doc/guix.texi

@@ -2358,9 +2358,10 @@ Run @var{mval}, a monadic value in the store monad, in @var{store}, an
 open store connection.
 @end deffn
 
-@deffn {Monadic Procedure} text-file @var{name} @var{text}
+@deffn {Monadic Procedure} text-file @var{name} @var{text} [@var{references}]
 Return as a monadic value the absolute file name in the store of the file
-containing @var{text}, a string.
+containing @var{text}, a string.  @var{references} is a list of store items that the
+resulting text file refers to; it defaults to the empty list.
 @end deffn
 
 @deffn {Monadic Procedure} interned-file @var{file} [@var{name}] @

guix/store.scm

@@ -890,11 +890,13 @@ taking the store as its first argument."
 ;; Store monad operators.
 ;;
 
-(define* (text-file name text)
+(define* (text-file name text
+                    #:optional (references '()))
   "Return as a monadic value the absolute file name in the store of the file
-containing TEXT, a string."
+containing TEXT, a string.  REFERENCES is a list of store items that the
+resulting text file refers to; it defaults to the empty list."
   (lambda (store)
-    (values (add-text-to-store store name text '())
+    (values (add-text-to-store store name text references)
             store)))
 
 (define* (interned-file file #:optional name