Updates to Macros and Customization chapters of Lisp manual.

Updates to Macros and Customization chapters of Lisp manual.

* doc/lispref/customize.texi (Common Keywords): Minor clarifications.
Document custom-unlispify-remove-prefixes.
(Variable Definitions): Backquotes in defcustom seem to work fine
now.  Various other copyedits.

* doc/lispref/macros.texi (Expansion): Minor clarification.
(Backquote): Move node to eval.texi.
(Defining Macros): Move an example from Backquote node.
(Argument Evaluation): No need to mention Pascal.
(Indenting Macros): Add xref to Defining Macros.

* doc/lispref/eval.texi (Backquote): Move from macros.texi.

* lisp/custom.el (defcustom): Doc fix.