Document 'base64:' prefix support

[philwebb]

As noted in #41047 (comment), we are currently missing documentation for base64: prefix support.

[vpavic]

I think the documentation committed to close this is not valid for 3.3 as support for arbitrary base64-encoded Resource properties has only been added in 3.4 - see #41433.

[snicoll]

Oh, that's very confusing then knowing that the code is available in 3.3.x and registered in META-INF/spring.factories.

[snicoll]

I've adapted the doc for 3.3.x in 89ffe1a