Document `eval' changes.

Document `eval' changes.

* doc/lispref/eval.texi (Eval): Document the LEXICAL arg to eval.

* doc/lispref/variables.texi (Variables, Void Variables): Use "scoping rule"
terminology consistently.
(Variable Scoping): Add index entries, and use "dynamic scope"
terminology in place of "indefinite scope" to reduce confusion.
(Lexical Binding): Document lexical environment format.
(Using Lexical Binding): Add index entries for error messages.