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

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'.