Improve indexing of 'eval-defun' in ELisp manual

* doc/lispref/display.texi (Defining Faces):
* doc/lispref/debugging.texi (Explicit Debug):
* doc/lispref/customize.texi (Variable Definitions):
* doc/lispref/variables.texi (Defining Variables): Add index
entries for 'eval-defun'.  (Bug#32066)

doc/lispref/customize.texi

@@ -321,6 +321,7 @@ If a @code{defcustom} does not specify any @code{:group}, the last group
 defined with @code{defgroup} in the same file will be used.  This way, most
 @code{defcustom} do not need an explicit @code{:group}.
 
+@cindex @code{eval-defun}, and @code{defcustom} forms
 When you evaluate a @code{defcustom} form with @kbd{C-M-x} in Emacs Lisp
 mode (@code{eval-defun}), a special feature of @code{eval-defun}
 arranges to set the variable unconditionally, without testing whether

doc/lispref/debugging.texi

@@ -336,6 +336,7 @@ which is not currently set up to break on change.
 @cindex debugger, explicit entry
 @cindex force entry to debugger
 
+@cindex @code{eval-defun}, and explicit entry to debugger
   You can cause the debugger to be called at a certain point in your
 program by writing the expression @code{(debug)} at that point.  To do
 this, visit the source file, insert the text @samp{(debug)} at the

doc/lispref/display.texi

@@ -2496,6 +2496,7 @@ However, if the customizations are subsequently removed, the
 appearance of @var{face} will again be determined by its default face
 spec.
 
+@cindex @code{eval-defun}, and @code{defface} forms
 As an exception, if you evaluate a @code{defface} form with
 @kbd{C-M-x} in Emacs Lisp mode (@code{eval-defun}), a special feature
 of @code{eval-defun} overrides any custom face specs on the face,

doc/lispref/variables.texi

@@ -461,6 +461,7 @@ form occurs in a @code{let} form with lexical binding enabled), then
 @code{defvar} sets the dynamic value.  The lexical binding remains in
 effect until its binding construct exits.  @xref{Variable Scoping}.
 
+@cindex @code{eval-defun}, and @code{defvar} forms
 When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in
 Emacs Lisp mode (@code{eval-defun}), a special feature of
 @code{eval-defun} arranges to set the variable unconditionally, without