Make it easier to enable Abbrev mode by default

* lisp/cus-start.el: Make 'abbrev-mode' customizable.
* doc/emacs/abbrevs.texi (Abbrev Concepts):
* etc/NEWS:
* src/buffer.c (syms_of_buffer): <abbrev-mode>: Document
enabling Abbrev mode by default in all buffers by customizing
'abbrev-mode' to a non-nil value.

doc/emacs/abbrevs.texi

@@ -49,11 +49,13 @@ insert @samp{find outer otter.}.
 @cindex Abbrev mode
 @cindex mode, Abbrev
   Abbrevs expand only when Abbrev mode, a buffer-local minor mode, is
-enabled.  Disabling Abbrev mode does not cause abbrev definitions to
-be forgotten, but they do not expand until Abbrev mode is enabled
-again.  The command @kbd{M-x abbrev-mode} toggles Abbrev mode; with a
-numeric argument, it turns Abbrev mode on if the argument is positive,
-off otherwise.  @xref{Minor Modes}.
+enabled.  Disabling Abbrev mode does not cause abbrev definitions to be
+forgotten, but they do not expand until Abbrev mode is enabled again.
+The command @kbd{M-x abbrev-mode} toggles Abbrev mode in the current
+buffer; with a numeric argument, it turns Abbrev mode on if the argument
+is positive, off otherwise (@pxref{Minor Modes}).  To enable Abbrev mode
+by default in all buffers, customize the variable @code{abbrev-mode} to
+a non-@code{nil} value (@pxref{Easy Customization}).
 
   Abbrevs can have @dfn{mode-specific} definitions, active only in one major
 mode.  Abbrevs can also have @dfn{global} definitions that are active in

etc/NEWS

@@ -3102,6 +3102,13 @@ contains an encrypted file, is an empty buffer, is a hidden buffer, or
 is the "*scratch*" buffer.  If it cannot locate any acceptable buffers,
 it will begrudgingly use the scratch buffer.
 
+** Abbrev mode
+
++++
+*** You can now enable Abbrev mode by default using Easy Customization.
+Customize the variable 'abbrev-mode' to non-nil to enable Abbrev mode by
+default in all buffers.
+
 
 * New Modes and Packages in Emacs 31.1
 

lisp/cus-start.el

@@ -173,6 +173,13 @@ Leaving \"Default\" unchecked is equivalent with specifying a default of
 	      "24.1")
              (delete-auto-save-files auto-save boolean)
              (kill-buffer-delete-auto-save-files auto-save boolean "28.1")
+             (abbrev-mode abbrev boolean nil
+                          ;; Not `custom-set-minor-mode' because it is a
+                          ;; buffer-local minor mode.  Customizing it to
+                          ;; non-nil means enabling the mode in all
+                          ;; buffers which don't locally disable it.
+                          :initialize custom-initialize-default
+                          :set custom-set-default)
 	     ;; callint.c
 	     (mark-even-if-inactive editing-basics boolean)
 	     ;; callproc.c

src/buffer.c

@@ -5219,7 +5219,9 @@ Format with `format-mode-line' to produce a string value.  */);
 
   DEFVAR_PER_BUFFER ("abbrev-mode", &BVAR (current_buffer, abbrev_mode), Qnil,
 		     doc: /* Non-nil if Abbrev mode is enabled.
-Use the command `abbrev-mode' to change this variable.  */);
+Use the command `abbrev-mode' to change the value of this variable in
+the current buffer.  Customize this variable to non-nil to enable Abbrev
+mode by default in all buffers.  */);
 
   DEFVAR_PER_BUFFER ("fill-column", &BVAR (current_buffer, fill_column),
 		     Qintegerp,