Fix mutability glitches reported by Drew Adams

Fix mutability glitches reported by Drew Adams

See Bug#40693#32.
* doc/lispref/eval.texi (Self-Evaluating Forms, Backquote):
Say that these yield constant conses, vectors and strings,
not constant symbols.
* doc/lispref/objects.texi (Constants and Mutability): Say that an
attempt to modify a constant variable signals an error, instead of
saying that it has undefined behavior.