Document constant vs mutable objects better

Document constant vs mutable objects better

This patch builds on a suggested patch by Mattias Engdegård
and on further comments by Eli Zaretskii.
Original bug report by Kevin Vigouroux (Bug#40671).
* doc/lispintro/emacs-lisp-intro.texi (set & setq, Review)
(setcar, Lists diagrammed, Mail Aliases, Indent Tabs Mode):
setq is a special form, not a function or command.
* doc/lispintro/emacs-lisp-intro.texi (setcar):
* doc/lispref/lists.texi (Modifying Lists, Rearrangement):
* doc/lispref/sequences.texi (Sequence Functions)
(Array Functions, Vectors):
* doc/lispref/strings.texi (String Basics, Modifying Strings):
Mention mutable vs constant objects.
* doc/lispintro/emacs-lisp-intro.texi (setcar, setcdr)
(kill-new function, cons & search-fwd Review):
* doc/lispref/edebug.texi (Printing in Edebug):
* doc/lispref/keymaps.texi (Changing Key Bindings):
* doc/lispref/lists.texi (Setcar, Setcdr, Rearrangement)
(Sets And Lists, Association Lists, Plist Access):
* doc/lispref/sequences.texi (Sequence Functions)
(Array Functions):
* doc/lispref/strings.texi (Text Comparison):
Fix examples so that they do not try to change constants.