Improve mutability documentation

This change was inspired by comments from Štěpán Němec in:
https://lists.gnu.org/r/emacs-devel/2020-04/msg01063.html
* doc/lispref/objects.texi (Lisp Data Types): Mention mutability.
(Constants and mutability): New section.
* doc/lispintro/emacs-lisp-intro.texi (Lists diagrammed)
(Indent Tabs Mode): Improve wording.
* doc/lispref/eval.texi (Self-Evaluating Forms):
Say that they return constants.
* doc/lispref/lists.texi (Sets And Lists):
Fix memql mistake/confusion that I recently introduced.

1 files changed, 1 insertions, 0 deletions

diff --git a/doc/lispref/strings.texi b/doc/lispref/strings.texi
index 3acbf538dce..a4c9c2549c5 100644
--- a/doc/lispref/strings.texi
+++ b/doc/lispref/strings.texi
@@ -384,6 +384,7 @@ usual value is @w{@code{"[ \f\t\n\r\v]+"}}.
   You can alter the contents of a mutable string via operations
 described in this section.  However, you should not try to use these
 operations to alter the contents of a constant string.
+@xref{Constants and Mutability}.
 
   The most basic way to alter the contents of an existing string is with
 @code{aset} (@pxref{Array Functions}).  @code{(aset @var{string}