pretty-options.txt: --notes accepts a ref instead of treeish

Although `--notes=` accepts and handles a tree-ish just fine, it isn't a
common use-case for users to pass in bare trees. By having "treeish", it
makes it harder to click in users' minds that the argument here is the
same type as the `notes.displayRef` configuration variable, for example.

Change `treeish` to `ref` so that it will be easier for users to make
this connection.

Note that we don't completely lose the notion that `--notes=` can accept
a tree-ish. In git-notes.txt, we have

	It is also permitted for a notes ref to point directly to a tree
	object, in which case the history of the notes can be read with
	`git log -p -g <refname>`.

which means that a hardcore user who wants to take advantage of this
obscure use-case will be able to infer the connection and not be
completely left in the dark.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-options.txt

@@ -57,7 +57,7 @@ message by 4 spaces (i.e.  'medium', which is the default, 'full',
 and 'fuller').
 
 ifndef::git-rev-list[]
---notes[=<treeish>]::
+--notes[=<ref>]::
 	Show the notes (see linkgit:git-notes[1]) that annotate the
 	commit, when showing the commit log message.  This is the default
 	for `git log`, `git show` and `git whatchanged` commands when
@@ -68,8 +68,8 @@ By default, the notes shown are from the notes refs listed in the
 `core.notesRef` and `notes.displayRef` variables (or corresponding
 environment overrides). See linkgit:git-config[1] for more details.
 +
-With an optional '<treeish>' argument, use the treeish to find the notes
-to display.  The treeish can specify the full refname when it begins
+With an optional '<ref>' argument, use the ref to find the notes
+to display.  The ref can specify the full refname when it begins
 with `refs/notes/`; when it begins with `notes/`, `refs/` and otherwise
 `refs/notes/` is prefixed to form a full name of the ref.
 +
@@ -85,7 +85,7 @@ being displayed. Examples: "--notes=foo" will show only notes from
 	"--notes --notes=foo --no-notes --notes=bar" will only show notes
 	from "refs/notes/bar".
 
---show-notes[=<treeish>]::
+--show-notes[=<ref>]::
 --[no-]standard-notes::
 	These options are deprecated. Use the above --notes/--no-notes
 	options instead.