Add new prolog fact to documentation

Documentation for the prolog fact added with [1].

[1] https://gerrit-review.googlesource.com/c/gerrit/+/258270

Change-Id: Ie0d357eb0d0a11c979ca7dd3fdbad99b6e70b605
(cherry picked from commit 8e6c5a5)

Documentation/prolog-change-facts.txt

@@ -48,6 +48,9 @@ of them we must use a qualified name like `gerrit:change_branch(X)`.
 |`commit_message/1`   |`commit_message('Fix bug X').`
     |Commit message as string atom
 
+|`commit_parent_count/1`   |`commit_parent_count(1).`
+    |Number of parent commits of a commit. Can be used to detect merge commits
+
 |`commit_stats/3`   |`commit_stats(5,20,50).`
     |Number of files modified, number of insertions and the number of deletions.