sidebar: the new-tab button says what it is, given a row to say it on

The mark went bare because in the strip it sits at the end of a row of
tabs, where the company it keeps says what it does and columns are
scarce. A sidebar row is neither: it is a row of its own with columns
to spare, and a lone glyph on it reads as decoration -- the hint under
the pointer cannot help, since you have to suspect a thing is a button
before you hover it.

So with the room it reads '+ new tab', in the word hint_for already
gives this action rather than a second name for the same thing, and
falls back to the bare mark when the sidebar is too narrow. The strip
is untouched. The row fills and hits like a tab's, so hover and the
drop-target fill cover the row rather than the word on it.

1 files changed, 2 insertions, 1 deletions

diff --git a/docs/panes.md b/docs/panes.md
index 329e017..ab9bd08 100644
--- a/docs/panes.md
+++ b/docs/panes.md
@@ -62,7 +62,8 @@ tab_bar_chrome true   // frame it like a pane
 A **sidebar lists one tab per row** down the chosen edge, and the panes trade
 the strip's row for its columns. Everything the strip does travels with it:
 click to switch, double-click to rename in place, drag a pane onto a tab or
-onto the `+`, drag tabs to reorder. The pane count and the prefix badge move
+onto the `+` — which, given a row of its own and the columns to spare, spells
+itself out as `+ new tab`. The pane count and the prefix badge move
 to the sidebar's bottom rows; the [status line](config.md) along the bottom
 keeps the full width. A terminal too narrow to give up the columns falls back
 to the top row until it grows.