Update the Customization chapter of Emacs manual.

Update the Customization chapter of Emacs manual.

* doc/emacs/custom.texi (Customization Groups): Update example.
(Browsing Custom): Document the new search field.
(Changing a Variable): Update example for Emacs 24 changes.
Document Custom-set and Custom-save commands.
(Face Customization): Document Emacs 24 changes.  De-document
modify-face.
(Specific Customization): Mention customize-variable.
(Custom Themes): Add customize-themes, custom-theme-load-path,
custom-theme-directory, and describe-theme.
(Creating Custom Themes): New node.
(Examining): Mention M-:.

* doc/emacs/package.texi (Packages): Fix typo.