mention that whereis --all skips dead keys

I looked at all man pages with --all, and this was the only one that
needs to add this disclaimer. Others like get and drop obviously
won't operate on dead keys anyway, because a dead key does not have any
content located anywhere.

doc/git-annex-whereis.mdwn

@@ -38,6 +38,9 @@ received from remotes.
 * `--all` `-A`
 
   Show whereis information for all known keys.
+  
+  (Except for keys that have been marked as dead,
+  see [[git-annex-dead]](1).)
 
 * `--branch=ref`