Fix documentation on Fn::Base64 function

The documentation says that the 'Fn::Base64' function converts an
input string into Base64, while the implementation is actually a
pass-through.  This patch fixes the documentation to avoid confusions
caused to users.

Change-Id: I643e73f7ed258da06d09f17a3302a258e3303414
Closes-Bug: 1377323
Merge "Fix documentation on Fn::Base64 function"