Rewrite documentation of buffer display

Rewrite documentation of buffer display

* doc/emacs/windows.texi (Window Choice): Rewrite, replacing
references to older buffer display options with references to
and examples of buffer display actions.
(Temporary Displays): Rewrite display of *Completions*
example.

* doc/lispref/elisp.texi (Top): New Windows section
'Displaying Buffers'.
* doc/lispref/frames.texi (Child Frames): Adjust cross
reference.
* doc/lispref/windows.texi (Windows): New section 'Displaying
Buffers'.  Move sections 'Choosing Window', 'Display Action
Functions' and 'Choosing Window Options' there and adjust
namings.  Preferably write 'Buffer Display Action' instead of
'Display Action'.  More consistently use @w{} to make key
binding specifications unsplittable.
(Displaying Buffers): New section.
(Choosing Window): Make it a subsection of 'Displaying
Buffers'.  More explicitly describe how 'display-buffer'
compiles its list of action functions and the action alist.
(Buffer Display Action Functions): Rename from 'Display Action
Functions', make it a subsection of 'Displaying Buffers' and
rewrite it.  Elide more detailed descriptions of action alist
entries; these are now in the new section 'Buffer Display
Action Functions'.  Remove example.
(Buffer Display Action Alists): New subsection of 'Displaying
Buffers' giving a comprehensive description of recognized
action alist entries with appropriate indexing.  Contents were
partially moved here from the old 'Display Action Functions'
section.
(Choosing Window Options): Make it a subsection of 'Displaying
Buffers'.  Add examples of how to rewrite old buffer display
options with the help of buffer display actions.
(Precedence of Action Functions): New subsection of
'Displaying Buffers' explaining the execution order of action
functions with the help of a detailed example.
(The Zen of Buffer Display): New subsection of 'Displaying
Buffers' supplying guidelines on how to write and use buffer
display actions with examples.
(Side Windows, Displaying Buffers in Side Windows)
(Frame Layouts with Side Windows, Atomic Windows): Update
references to the 'Displaying Buffers' subsections.