doc: cookbook: Use @file for file names and extensions.

* doc/guix-cookbook.texi (@samp{GUIX_PACKAGE_PATH}):
(Extended example):
(Setting up a bind mount):
(Basic setup with manifests):
(Default profile): Use @file instead of @samp or @code for file names and
extensions.

doc/guix-cookbook.texi

@@ -581,7 +581,7 @@ packages.
 Guix makes it possible to streamline the process by adding as many ``package
 declaration directories'' as you want.
 
-Create a directory, say @samp{~./guix-packages} and add it to the @samp{GUIX_PACKAGE_PATH}
+Create a directory, say @file{~./guix-packages} and add it to the @samp{GUIX_PACKAGE_PATH}
 environment variable:
 
 @example
@@ -851,7 +851,7 @@ version when packaging programs for a specific commit.
 @subsubsection Snippets
 
 Snippets are quoted (i.e. non-evaluated) Scheme code that are a means of patching
-the source.  They are a Guix-y alternative to the traditional @samp{.patch} files.
+the source.  They are a Guix-y alternative to the traditional @file{.patch} files.
 Because of the quote, the code in only evaluated when passed to the Guix daemon
 for building.  There can be as many snippets as needed.
 
@@ -955,7 +955,7 @@ $ make CC=gcc prefix=/gnu/store/...-<out>
 This sets the C compiler to @code{gcc} and the @code{prefix} variable (the installation
 directory in Make parlance) to @code{(assoc-ref %outputs "out")}, which is a build-stage
 global variable pointing to the destination directory in the store (something like
-@samp{/gnu/store/...-my-libgit2-20180408}).
+@file{/gnu/store/...-my-libgit2-20180408}).
 
 Similarly, it's possible to set the configure flags:
 
@@ -1622,8 +1622,8 @@ Then you need to add the following code to a StumpWM configuration file
 To bind mount a file system, one must first set up some definitions
 before the @code{operating-system} section of the system definition.  In
 this example we will bind mount a folder from a spinning disk drive to
-@code{/tmp}, to save wear and tear on the primary SSD, without
-dedicating an entire partition to be mounted as @code{/tmp}.
+@file{/tmp}, to save wear and tear on the primary SSD, without
+dedicating an entire partition to be mounted as @file{/tmp}.
 
 First, the source drive that hosts the folder we wish to bind mount
 should be defined, so that the bind mount can depend on it.
@@ -1801,9 +1801,9 @@ Note that it's also possible to loop over the output of
 guix package --list-profiles
 @end example
 
-although you'll probably have to filter out @samp{~/.config/guix/current}.
+although you'll probably have to filter out @file{~/.config/guix/current}.
 
-To enable all profiles on login, add this to your @samp{~/.bash_profile} (or similar):
+To enable all profiles on login, add this to your @file{~/.bash_profile} (or similar):
 
 @example
 for i in $GUIX_EXTRA_PROFILES/*; do
@@ -1817,8 +1817,8 @@ done
 @end example
 
 Note to Guix System users: the above reflects how your default profile
-@samp{~/.guix-profile} is activated from @samp{/etc/profile}, that latter being loaded by
-@samp{~/.bashrc} by default.
+@file{~/.guix-profile} is activated from @file{/etc/profile}, that latter being loaded by
+@file{~/.bashrc} by default.
 
 You can obviously choose to only enable a subset of them:
 
@@ -1861,7 +1861,7 @@ guix package -m /path/to/guix-my-project-manifest.scm -p "$GUIX_EXTRA_PROFILES"/
 
 To upgrade all profiles, it's easy enough to loop over them.  For instance,
 assuming your manifest specifications are stored in
-@samp{~/.guix-manifests/guix-$profile-manifest.scm}, with @samp{$profile} being the name
+@file{~/.guix-manifests/guix-$profile-manifest.scm}, with @samp{$profile} being the name
 of the profile (e.g.@: "project1"), you could do the following in Bourne shell:
 
 @example
@@ -1921,7 +1921,7 @@ The same is true for @samp{INFOPATH} (you can install @samp{info-reader}),
 @node Default profile
 @subsection Default profile
 
-What about the default profile that Guix keeps in @samp{~/.guix-profile}?
+What about the default profile that Guix keeps in @file{~/.guix-profile}?
 
 You can assign it the role you want.  Typically you would install the manifest
 of the packages you want to use all the time.