doc: Document 'guix-edit' Emacs command.

* doc/emacs.texi (Emacs Commands): Document "M-x guix-edit".
* doc/guix.texi (Invoking guix edit): Mention it.

doc/emacs.texi

@@ -219,6 +219,28 @@ With @kbd{C-u}, make it verbose.
 Once @command{guix pull} has succeeded, the Guix REPL is restared.  This
 allows you to keep using the Emacs interface with the updated Guix.
 
+Finally, there is an Emacs variant of @command{guix edit} command
+(@pxref{Invoking guix edit}):
+
+@table @kbd
+@item M-x guix-edit
+As with @kbd{M-x guix-packages-by-name}, you can press @key{TAB} to
+complete a package name.
+@end table
+
+If you are contributing to Guix, you may find it useful for @kbd{M-x
+guix-edit} to open package files from your git directory.  This can be
+done by setting @code{guix-directory} variable.  For example, after
+this:
+
+@example
+(setq guix-directory "~/src/guix")
+@end example
+
+@kbd{M-x guix-edit guix} opens
+@file{~/src/guix/gnu/packages/package-management.scm} file.
+
+
 @node Emacs General info
 @subsection General information
 

doc/guix.texi

@@ -4201,9 +4201,9 @@ launches the program specified in the @code{VISUAL} or in the
 and that of Vim.
 
 If you are using Emacs, note that the Emacs user interface provides
-similar functionality in the ``package info'' and ``package list''
-buffers created by @kbd{M-x guix-search-by-name} and similar commands
-(@pxref{Emacs Commands}).
+@kbd{M-x guix-edit} command and a similar functionality in the ``package
+info'' and ``package list'' buffers created by @kbd{M-x
+guix-search-by-name} and similar commands (@pxref{Emacs Commands}).
 
 
 @node Invoking guix download