Don’t use “constant” for values you shouldn’t change

Don’t use “constant” for values you shouldn’t change

Inspired by patch proposed by Dmitry Gutov (Bug#40671#393)
and by further comments by him and by Michael Heerdegen
in the same bug report.
* doc/lispintro/emacs-lisp-intro.texi (setcar):
Don’t push mutability here.
* doc/lispref/eval.texi (Self-Evaluating Forms, Quoting)
(Backquote):
* doc/lispref/lists.texi (Modifying Lists):
* doc/lispref/objects.texi (Lisp Data Types, Mutability):
* doc/lispref/sequences.texi (Array Functions, Vectors):
* doc/lispref/strings.texi (String Basics, Modifying Strings):
Don’t use the word “constant” to describe all values that
a program should not change.
* doc/lispref/objects.texi (Mutability):
Rename from “Constants and Mutability”.  All uses changed.
In a footnote, contrast the Emacs behavior with that of Common
Lisp, Python, etc. for clarity, and say the goal is to be nicer.