doc: Add JFS to Guix System-supported file systems.

* doc/guix.texi (Keyboard Layout and Networking and Partitioning):
Note JFS support.

doc/guix.texi

@@ -49,7 +49,7 @@ Copyright @copyright{} 2017 Christopher Allan Webber@*
 Copyright @copyright{} 2017, 2018, 2019 Marius Bakke@*
 Copyright @copyright{} 2017, 2019 Hartmut Goebel@*
 Copyright @copyright{} 2017, 2019 Maxim Cournoyer@*
-Copyright @copyright{} 2017, 2018, 2019 Tobias Geerinckx-Rice@*
+Copyright @copyright{} 2017, 2018, 2019, 2020 Tobias Geerinckx-Rice@*
 Copyright @copyright{} 2017 George Clemmer@*
 Copyright @copyright{} 2017 Andy Wingo@*
 Copyright @copyright{} 2017, 2018, 2019 Arun Isaac@*
@@ -2245,8 +2245,8 @@ bootloaders.
 Once you are done partitioning the target hard disk drive, you have to
 create a file system on the relevant partition(s)@footnote{Currently
-Guix System only supports ext4 and btrfs file systems.  In particular, code
-that reads file system UUIDs and labels only works for these file system
+Guix System only supports ext4, btrfs, and JFS file systems.  In particular,
+code that reads file system UUIDs and labels only works for these file system
 types.}.  For the ESP, if you have one and assuming it is
 @file{/dev/sda1}, run: