Improve documentation of 'header-line-indent-mode'

* doc/lispref/modes.texi (Header Lines): Rewrite the documentation
of 'header-line-indent-mode' and its two variables.  Fix the
example.
* doc/lispref/display.texi (Pixel Specification): More accurate
description of what happens with :align-to in header-lines.
Improve indexing.  (Bug#61239)

* src/buffer.c (syms_of_buffer) <header-line-format>:
* lisp/display-line-numbers.el (header-line-indent)
(header-line-indent-width, header-line-indent-mode): Doc fixes.

* etc/NEWS: Enhance the announcement of 'header-line-indent-mode'.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/buffer.c b/src/buffer.c
index 88ca69b0dd8..38648519ba0 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -5081,8 +5081,8 @@ the mode line appears at the bottom.  */);
 The header line appears, optionally, at the top of a window; the mode
 line appears at the bottom.
 
-Also see `header-line-indent-mode' if `display-line-number-mode' is
-used.  */);
+Also see `header-line-indent-mode' if `display-line-numbers-mode' is
+turned on and header-line text should be aligned with buffer text.  */);
 
   DEFVAR_PER_BUFFER ("mode-line-format", &BVAR (current_buffer, mode_line_format),
                      Qnil,