Documentation: commit-tree: remove 16 parents restriction

ef98c5ca lifted the 16 parents restriction in builtin-commit-tree.c,
but forgot to update the documentation.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit-tree.txt

@@ -16,12 +16,12 @@ This is usually not what an end user wants to run directly.  See
 linkgit:git-commit[1] instead.
 
 Creates a new commit object based on the provided tree object and
-emits the new commit object id on stdout. If no parent is given then
-it is considered to be an initial tree.
+emits the new commit object id on stdout.
 
-A commit object usually has 1 parent (a commit after a change) or up
-to 16 parents.  More than one parent represents a merge of branches
-that led to them.
+A commit object may have any number of parents. With exactly one
+parent, it is an ordinary commit. Having more than one parent makes
+the commit a merge between several lines of history. Initial (root)
+commits have no parents.
 
 While a tree represents a particular directory state of a working
 directory, a commit represents that state in "time", and explains how