(Split Window): Document integer values of

truncate-partial-width-windows.

doc/emacs/windows.texi

@@ -107,12 +107,20 @@ scroll bar, the mode line of the split window is side by side with
 your click.
 
 @vindex truncate-partial-width-windows
-  When a window is less than the full width, text lines too long to
-fit are frequent.  Continuing all those lines might be confusing, so
-if the variable @code{truncate-partial-width-windows} is
-non-@code{nil}, that forces truncation in all windows less than the
-full width of the screen, independent of the buffer being displayed
-and its value for @code{truncate-lines}.  @xref{Line Truncation}.
+  When a window occupies less than the full width of the frame, it may
+become too narrow for most of the text lines in its buffer.  If most
+of its lines are continued, the buffer may become difficult to read.
+Therefore, Emacs automatically truncates lines if the window width
+becomes narrower than 50 columns.  This truncation occurs regardless
+of the value of the variable @code{truncate-lines} (@pxref{Line
+Truncation}); it is instead controlled by the variable
+@code{truncate-partial-width-windows}.  If the value of
+@code{truncate-partial-width-windows} is a positive integer (the
+default is 50), that specifies the minimum width for a partial-width
+window before automatic line truncation occurs; if the value is
+@code{nil}, automatic line truncation is disabled; and for any other
+non-@code{nil} value, Emacs truncates lines in every partial-width
+window regardless of its width.
 
   Horizontal scrolling is often used in side-by-side windows.
 @xref{Horizontal Scrolling}.