Mention the fact that 'git annotate' is only for backward compatibility.

When somebody is reading git-blame.txt (or git-annotate.txt) for the first
time, the message we would like to send is:

 (1) Here is why you would want to use this command, what it can do
     (perhaps more than what you would have expected from "$scm blame"),
     and how you tell it to do what it does.

     This is obvious.

 (2) You might have heard of the command with the other name.  There is no
     difference between the two, except they differ in their default
     output formats.

     This is essential to answer: "git has both?  how are they different?"

 (3) We tend to encourage blame over annotate for new scripts and new
     people, but there is no reason to choose one over the other.

     This is not as important as (2), but would be useful to avoid
     repeated questions about "when will we start deprecating this?"

As long as we describe (2) on git-annotate page clearly enough, people who
read git-blame page first and get curious can refer to git-annotate page.
While at it, subtly hint (3) without being overly explicit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-annotate.txt

@@ -14,6 +14,11 @@ DESCRIPTION
 Annotates each line in the given file with information from the commit
 which introduced the line. Optionally annotate from a given revision.
 
+The only difference between this command and linkgit:git-blame[1] is that
+they use slightly different output formats, and this command exists only
+for backward compatibility to support existing scripts, and provide more
+familiar command name for people coming from other SCM systems.
+
 OPTIONS
 -------
 include::blame-options.txt[]