git blame: document that it always follows origin across whole-file renames

Make it clear to people who (rightly or wrongly) think that the
"--follow" option should follow origin across while-file renames
that we already do so.  That would explain the output that they see
when they do give the "--follow" option to the command.

We may or may not want to do a "--no-follow" patch as a follow-up,
but that is a separate topic.

Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

Documentation/git-blame.txt

@@ -20,6 +20,12 @@ last modified the line. Optionally, start annotating from the given revision.
 
 The command can also limit the range of lines annotated.
 
+The origin of lines is automatically followed across whole-file
+renames (currently there is no option to turn the rename-following
+off). To follow lines moved from one file to another, or to follow
+lines that were copied and pasted from another file, etc., see the
+`-C` and `-M` options.
+
 The report does not tell you anything about lines which have been deleted or
 replaced; you need to use a tool such as 'git diff' or the "pickaxe"
 interface briefly mentioned in the following paragraph.