Fix documentation of 'undo' changes

* doc/lispref/text.texi (Undo): Minor wording changes.  Use US
English conventions for spelling and whitespace between sentences.

* etc/NEWS: Fix wording and spelling of undo-related entries.
Mark them as documented.

doc/lispref/text.texi

@@ -1353,26 +1353,27 @@ appropriate time.
 @end defun
 
 @defun undo-auto-amalgamate
+@cindex amalgamating commands, and undo
 The editor command loop automatically calls @code{undo-boundary} just
 before executing each key sequence, so that each undo normally undoes
 the effects of one command.  A few exceptional commands are
-@emph{amalgamating}: these commands generally cause small changes to
-buffers. So with these a boundary is inserted only every 20th command,
-so that these can be undone as a group.  By default commands
+@dfn{amalgamating}: these commands generally cause small changes to
+buffers, so with these a boundary is inserted only every 20th command,
+allowing to undo them as a group.  By default, commands
 @code{self-insert-command}, which produces self-inserting input
 characters (@pxref{Commands for Insertion}), and @code{delete-char}
 which deletes characters (@pxref{Deletion}) are amalgamating.
-Where a command affects the contents of several buffers as may happen,
-for example, if a function on the @code{post-command-hook} affects a
+Where a command affects the contents of several buffers, as may happen,
+for example, when a function on the @code{post-command-hook} affects a
 buffer other than the @code{current-buffer}, then @code{undo-boundary}
-will be called in each of them.
+will be called in each of the affected buffers.
 @end defun
 
 @defvar undo-auto-current-boundary-timer
 Some buffers, such as process buffers, can change even when no
-commands are executing. In these cases, @code{undo-boundary} is
-normally called periodically by the timer in this variable. Setting
-this variable to non-@code{nil} prevents this behaviour.
+commands are executing.  In these cases, @code{undo-boundary} is
+normally called periodically by the timer in this variable.  Setting
+this variable to non-@code{nil} prevents this behavior.
 @end defvar
 
 @defvar undo-in-progress

etc/NEWS

@@ -226,12 +226,19 @@ at configure time.
 +++
 ** M-x suggests shorthands and ignores obsolete commands for completion.
 
-** Successive single-char deletions are collapsed in the undo-log just like
-successive char insertions. This behaviour can be extended to other
-commands, using the `undo-auto--amalgamate' function.
+** Changes in undo
 
-** The heuristic used to insert `undo-boundary' after each command has changed,
-so that it supports commands which potentially affect multiple buffers.
++++
+*** Successive single-char deletions are collapsed in the undo-log just like
+successive char insertions.  Which commands invoke this behavior is
+controlled by the new `undo-auto-amalgamate' function.  See the node
+"Undo" in the ELisp manual for more details.
+
++++
+*** The heuristic used to insert `undo-boundary' after each command
+has changed, so that if a command causes changes in more than just the
+current buffer, Emacs now calls `undo-boundary' in every buffer
+affected by the command.
 
 +++
 ** New command `comment-line' bound to `C-x C-;'.