Document git-svn's first-parent rule

git-svn has the following rule to detect the SVN base for its
operations: find the first git-svn-id line reachable through
first-parent ancestry.  IOW,

  git log --grep=^git-svn-id: --first-parent -1

Document this, as it is very important when using merges with git-svn.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>

Documentation/git-svn.txt

@@ -742,6 +742,16 @@ merges you've made.  Furthermore, if you merge or pull from a git branch
 that is a mirror of an SVN branch, 'dcommit' may commit to the wrong
 branch.
 
+If you do merge, note the following rule: 'git svn dcommit' will
+attempt to commit on top of the SVN commit named in
+------------------------------------------------------------------------
+git log --grep=^git-svn-id: --first-parent -1
+------------------------------------------------------------------------
+You 'must' therefore ensure that the most recent commit of the branch
+you want to dcommit to is the 'first' parent of the merge.  Chaos will
+ensue otherwise, especially if the first parent is an older commit on
+the same SVN branch.
+
 'git clone' does not clone branches under the refs/remotes/ hierarchy or
 any 'git svn' metadata, or config.  So repositories created and managed with
 using 'git svn' should use 'rsync' for cloning, if cloning is to be done