Fix inaccurate wording in the Emacs manual

* doc/emacs/custom.texi (Modifier Keys):
* doc/emacs/basic.texi (Inserting Text): Fix minor inaccuracies in
describing the 'Alt' modifier.  (Bug#39254)

Author: Eli-Zaretskii

doc/emacs/basic.texi

@@ -117,9 +117,9 @@ which is Unicode code-point U+2018 @sc{left single quotation mark},
 sometimes called a left single ``curved quote'' or ``curly quote''.
 Similarly, @kbd{C-x 8 ]}, @kbd{C-x 8 @{} and @kbd{C-x 8 @}} insert the
 curved quotes @t{’}, @t{“} and @t{”}, respectively.  Also, a working
-Alt key acts like @kbd{C-x 8}; e.g., @kbd{A-[} acts like @kbd{C-x 8 [}
-and inserts @t{‘}.  To see which characters have @kbd{C-x 8}
-shorthands, type @kbd{C-x 8 C-h}.
+@key{Alt} key acts like @kbd{C-x 8} (unless followed by @key{RET});
+e.g., @kbd{A-[} acts like @kbd{C-x 8 [} and inserts @t{‘}.  To see
+which characters have @kbd{C-x 8} shorthands, type @kbd{C-x 8 C-h}.
 
   Alternatively, you can use the command @kbd{C-x 8 @key{RET}}
 (@code{insert-char}).  This prompts for the Unicode name or code-point

doc/emacs/custom.texi

@@ -1930,10 +1930,12 @@ commonly used, Emacs supports three other modifier keys.  These are
 called @key{Super}, @key{Hyper}, and @key{Alt}.  Few terminals provide
 ways to use these modifiers; the key labeled @key{Alt} on most
 keyboards usually issues the @key{Meta} modifier, not @key{Alt}.  The
-standard key bindings in Emacs do not include any characters with
-these modifiers.  However, you can customize Emacs to assign meanings
-to them.  The modifier bits are labeled as @samp{s-}, @samp{H-} and
-@samp{A-} respectively.
+standard key bindings in Emacs do not include any characters with the
+@key{Super} and @key{Hyper} modifiers, and only a small number of
+standard key bindings use @key{Alt}.  However, you can customize Emacs
+to assign meanings to key bindings that use these modifiers.  The
+modifier bits are labeled as @samp{s-}, @samp{H-} and @samp{A-}
+respectively.
 
   Even if your keyboard lacks these additional modifier keys, you can
 enter it using @kbd{C-x @@}: @kbd{C-x @@ h} adds the Hyper flag to