Update docs for window dividers and `window-text-pixel-size'.

Update docs for window dividers and `window-text-pixel-size'.

* xdisp.c (Fwindow_text_pixel_size): Adjust doc-string.
* elisp.texi (Top): Rename section "Width" to "Size of Displayed
Text".
* text.texi (Primitive Indent):
* strings.texi (String Basics):
* sequences.texi (Sequence Functions): Update references
accordingly.
* display.texi (Size of Displayed Text): Rename section from
"Width".  Add description for `window-text-pixel-size'.
(Window Dividers): Reword description of window dividers.
* frames.texi (Layout Parameters): Improve description of window
divider parameters.
* windows.texi (Window Sizes): Add descriptions of
`window-mode-line-height' and `window-header-line-height'.
(Coordinates and Windows): Mention window dividers.