gnu: mongodb: Add comment about future package versions.

* gnu/packages/databases.scm (mongodb): Add comment that the license of
the package changed after the packaged version and that it shouldn't be
updated.

gnu/packages/databases.scm

@@ -661,6 +661,9 @@ (define-public python-pylibmc
 (define-public python2-pylibmc
   (package-with-python2 python-pylibmc))
 
+;; This is the last version of mongodb before they switched to the Server Side
+;; Public License (SSPL), which is not a FOSS license.  As such, this package
+;; cannot be updated.
 (define-public mongodb
   (package
     (name "mongodb")