Document 'nomergebasebranch' build option.

wiki/src/contribute/build.mdwn

@@ -195,10 +195,8 @@ way:
    repository. By default, it will refuse to start a build in presence of
    uncommitted changes.
 
- * **mergebasebranch**: if building from a branch (not tag!) this
-   forces a merge of the base branch before the build process starts
-   for real. This is mostly meant for our Jenkins deployment, so use
-   at your own risk.
+ * **nomergebasebranch**: by default the base branch is merged before
+   building, and this option overrides that behavior.
 
 ## Variations useful for testing build reproducibility