Update Variables chapter of Lisp manual to handle lexical binding.

Update Variables chapter of Lisp manual to handle lexical binding.

* doc/lispref/variables.texi (Variables, Local Variables, Void Variables):
Edit to make the descriptions less specific to dynamic binding.
(Local Variables): Default max-specpdl-size is now 1300.
(Defining Variables): Edits for lexical scoping.  Delete
information about starting docstrings with *.  De-document
user-variable-p.
(Tips for Defining): Remove an unimportant discussion of quitting
in the middle of a load.
(Accessing Variables, Setting Variables): Discuss lexical binding.
(Variable Scoping): Rewrite.
(Scope, Extent, Impl of Scope): Nodes deleted.
(Dynamic Binding): New node, with material from Scope, Extent, and
Impl of Scope nodes.
(Dynamic Binding Tips): Rename from Using Scoping.
(Lexical Binding): Rewrite.
(Using Lexical Binding): Rename from Converting to Lexical
Binding.  Convert to subsection.

* doc/lispref/customize.texi (Variable Definitions): Add custom-variable-p.
Move user-variable-p documentation here.