Improve documentatuon of 'truncate-partial-width-windows'

* src/xdisp.c (syms_of_xdisp) <truncate-partial-width-windows>:
Clarify in the doc string how the width of partial-width windows
is computed for the purposes of truncation decision.  (Bug#4338)

* doc/emacs/windows.texi (Split Window): Clarify how the width of
windows is calculated for the purposes of truncation decision.
* doc/emacs/display.texi (Line Truncation): Remove a redundant
index entry.

1 files changed, 5 insertions, 2 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index d6ee2de8f39..d701306b22c 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -31422,8 +31422,11 @@ Value is a number or a cons (WIDTH-DPI . HEIGHT-DPI).  */);
                Vtruncate_partial_width_windows,
     doc: /* Non-nil means truncate lines in windows narrower than the frame.
 For an integer value, truncate lines in each window narrower than the
-full frame width, provided the window width is less than that integer;
-otherwise, respect the value of `truncate-lines'.
+full frame width, provided the total window width in column units is less
+than that integer; otherwise, respect the value of `truncate-lines'.
+The total width of the window is as returned by `window-total-width', it
+includes the fringes, the continuation and truncation glyphs, the
+display margins (if any), and the scroll bar
 
 For any other non-nil value, truncate lines in all windows that do
 not span the full frame width.