Correct documentation of 'reflog show' to explain it shows HEAD

By default 'git reflog show' will show the reflog of 'HEAD' and not
the reflog of the current branch.  This is most likely due to the
work done a while ago as part of the detached HEAD series to allow
HEAD to have its own reflog independent of each branch's reflog.

Since 'git reflog show' is really just an obscure alias for 'git
log -g --abbrev-commit --pretty=oneline' it should behave the same
way and its documentation should match.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-reflog.txt

@@ -32,7 +32,8 @@ directly by the end users -- instead, see gitlink:git-gc[1].
 
 The subcommand "show" (which is also the default, in the absense of any
 subcommands) will take all the normal log options, and show the log of
-the current branch. It is basically an alias for 'git log -g --abbrev-commit
+`HEAD`, which will cover all recent actions, including branch switches.
+It is basically an alias for 'git log -g --abbrev-commit
 --pretty=oneline', see gitlink:git-log[1].