Documentation/git-rev-parse.txt: Clarify minimum length=4

Here I merely repeat "The minimum length is 4." from another part of the page. However I do it in a critical spot, else users will think that there is just no dress code for how short one's strings can be.

Proof: only the last line of:
git init
touch z
git add z
git cat-file -t e
git cat-file -t e6
git cat-file -t e69
git cat-file -t e69d
will finally output "blob".

---
 Documentation/git-rev-parse.txt |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

index 2921da3..8a4cdfc 100644
@@ -154,6 +154,7 @@ blobs contained in a commit.
   E.g. dae86e1950b1277e545cee180551750029cfe735 and dae86e both
   name the same commit object if there are no other object in
   your repository whose object name starts with dae86e.
+  The minimum length is 4.
 
 * An output from 'git-describe'; i.e. a closest tag, optionally
   followed by a dash and a number of commits, followed by a dash, a

1.6.0.6