Update several Lisp manual chapters.

Update several Lisp manual chapters.

* doc/lispref/eval.texi (Intro Eval, Symbol Forms): Minor tweaks for
correctness with lexical scoping.
(Eval): Copyedits.

* doc/lispref/sequences.texi (Sequence Functions): Don't repeat the
introduction already given in the parent.
(Vectors): Copyedits.
(Rings): Move from lists.texi.  Note that this is specific to the
ring package.

* doc/lispref/lists.texi (Cons Cells): Copyedits.
(List Elements): Mention push.
(List Variables): Mention pop.
(Rings): Move to sequences.texi.

* doc/lispref/strings.texi (Text Comparison): Minor qualification.

* doc/lispref/symbols.texi (Definitions, Symbol Components): Mention variable
scoping issues.
(Plists and Alists): Copyedits.