* doc/emacs/frames.texi (Tab Bars): Describe tab-last.

'C-9' and 'M-9' already had kindex, so describe them in the text as well
(bug#51444).

doc/emacs/frames.texi

@@ -1456,9 +1456,7 @@ names is sorted by recency, so you can use @kbd{M-n}
 the second last, and so on.
 
 @kindex C-1, tab bar
-@kindex C-9, tab bar
 @kindex M-1, tab bar
-@kindex M-9, tab bar
 @findex tab-select
 @vindex tab-bar-select-tab-modifiers
 @vindex tab-bar-tab-hints
@@ -1474,6 +1472,15 @@ name, you can customize another variable @code{tab-bar-tab-hints}.
 This will help you decide which numerical key to press to select the
 tab by its number.
 
+@kindex C-9, tab bar
+@kindex M-9, tab bar
+@findex tab-last
+@item @var{modifier}-@kbd{9}
+Switch to the last tab (@code{tab-last}).  The key combination is
+the modifier key defined by @code{tab-bar-select-tab-modifiers} and
+the key @kbd{9}.  With a numeric argument @var{n}, switch to the
+@var{n}th last tab.
+
 @kindex C-0, tab bar
 @kindex M-0, tab bar
 @findex tab-recent

etc/NEWS

@@ -515,14 +515,14 @@ aligned to the right on the tab bar instead of on the mode line.
 When 'tab-bar-format-tabs' is replaced with 'tab-bar-format-tabs-groups',
 the tab bar displays tab groups.
 
----
++++
 *** New optional key binding for 'tab-last'.
 If you customize the user option 'tab-bar-select-tab-modifiers' to
 allow selecting tabs using their index numbers, the '<MODIFIER>-9' key
 is bound to 'tab-last', and switches to the last tab.  Here <MODIFIER>
 is any of the modifiers in the list that is the value of
-'tab-bar-select-tab-modifiers'.  You can also use negative indices,
-which count from the last tab: -1 is the last tab, -2 the one before
+'tab-bar-select-tab-modifiers'.  You can also use positive indices,
+which count from the last tab: 1 is the last tab, 2 the one before
 that, etc.
 
 ---