Rename API and reference doc output dirs

Prior to this change, aggregated javadoc has been written to an 'api'
dir and reference docs into a 'reference' dir. This shorter naming is
desirable, but is not compatible with historical naming under 3.1.x as
defined by the Ant+Ivy Spring Build.

For example, URLs are currently as follows:

http://static.springsource.org/spring-framework/docs/3.1.3.RELEASE/javadoc-api
http://static.springsource.org/spring-framework/docs/3.1.x/javadoc-api
http://static.springsource.org/spring-framework/docs/current/javadoc-api

Particularly with regard to the 'current' link, we do not want to
break this url scheme with the shortened 'api' directory naming. Of
course this compatibility can be preserved with symlinks, but this makes
the release process that much more complicated and fragile.

This commit ensures that the naming is 'javadoc-api' and
'spring-framework-reference', consistent with historical convention.
We can always add a symlinking scheme to add the shorter 'api' and
'reference' options after the fact if desired.

build.gradle

@@ -733,11 +733,11 @@ configure(rootProject) {
 		}
 
 		from (api) {
-			into "api"
+			into "javadoc-api"
 		}
 
 		from (reference) {
-			into "reference"
+			into "spring-framework-reference"
 		}
 	}