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, 2 insertions, 2 deletions

diff --git a/doc/lispintro/emacs-lisp-intro.texi b/doc/lispintro/emacs-lisp-intro.texi
index 630676d9786..ea16d9ef155 100644
--- a/doc/lispintro/emacs-lisp-intro.texi
+++ b/doc/lispintro/emacs-lisp-intro.texi
@@ -9557,7 +9557,7 @@ part of which is the address of the next pair.  The very last box
 points to the symbol @code{nil}, which marks the end of the list.
 
 @need 1200
-When a variable is set to a list via @code{setq},
+When a variable is set to a list with an operation such as @code{setq},
 it stores the address of the first box in the variable.  Thus,
 evaluation of the expression
 
@@ -17140,7 +17140,7 @@ The following turns off Indent Tabs mode:
 @end smallexample
 
 Note that this line uses @code{setq-default} rather than the
-@code{setq} that we have seen before.  The @code{setq-default}
+@code{setq} that we have seen before; @code{setq-default}
 sets values only in buffers that do not have their own local
 values for the variable.