guix: Clean up --help messages.

* guix/scripts/import.scm (show-help): Add newline before a list of
  importers.
* guix/scripts/lint.scm (show-help): Split a long description line.
* guix/scripts/package.scm (show-help): Improve docstrings for --install
  and --remove options.
* guix/scripts/system.scm (show-help): Format actions the same way as
  guix commands and importers are formatted.

guix/scripts/import.scm

@@ -86,6 +86,7 @@ rather than \\n."
 Run IMPORTER with ARGS.\n"))
   (newline)
   (display (_ "IMPORTER must be one of the importers listed below:\n"))
+  (newline)
   (format #t "~{   ~a~%~}" importers)
   (display (_ "
   -h, --help             display this help and exit"))

guix/scripts/lint.scm

@@ -575,7 +575,8 @@ descriptions maintained upstream."
 
 (define (show-help)
   (display (_ "Usage: guix lint [OPTION]... [PACKAGE]...
-Run a set of checkers on the specified package; if none is specified, run the checkers on all packages.\n"))
+Run a set of checkers on the specified package; if none is specified,
+run the checkers on all packages.\n"))
   (display (_ "
   -c, --checkers=CHECKER1,CHECKER2...
                          only run the specificed checkers"))

guix/scripts/package.scm

@@ -422,15 +422,17 @@ ENTRIES, a list of manifest entries, in the context of PROFILE."
     (substitutes? . #t)))
 
 (define (show-help)
-  (display (_ "Usage: guix package [OPTION]... PACKAGES...
-Install, remove, or upgrade PACKAGES in a single transaction.\n"))
+  (display (_ "Usage: guix package [OPTION]...
+Install, remove, or upgrade packages in a single transaction.\n"))
   (display (_ "
-  -i, --install=PACKAGE  install PACKAGE"))
+  -i, --install PACKAGE ...
+                         install PACKAGEs"))
   (display (_ "
   -e, --install-from-expression=EXP
                          install the package EXP evaluates to"))
   (display (_ "
-  -r, --remove=PACKAGE   remove PACKAGE"))
+  -r, --remove PACKAGE ...
+                         remove PACKAGEs"))
   (display (_ "
   -u, --upgrade[=REGEXP] upgrade all the installed packages matching REGEXP"))
   (display (_ "

guix/scripts/system.scm

@@ -368,18 +368,19 @@ boot directly to the kernel or to the bootloader."
 Build the operating system declared in FILE according to ACTION.\n"))
   (newline)
   (display (_ "The valid values for ACTION are:\n"))
+  (newline)
   (display (_ "\
-  - 'reconfigure', switch to a new operating system configuration\n"))
+   reconfigure      switch to a new operating system configuration\n"))
   (display (_ "\
-  - 'build', build the operating system without installing anything\n"))
+   build            build the operating system without installing anything\n"))
   (display (_ "\
-  - 'vm', build a virtual machine image that shares the host's store\n"))
+   vm               build a virtual machine image that shares the host's store\n"))
   (display (_ "\
-  - 'vm-image', build a freestanding virtual machine image\n"))
+   vm-image         build a freestanding virtual machine image\n"))
   (display (_ "\
-  - 'disk-image', build a disk image, suitable for a USB stick\n"))
+   disk-image       build a disk image, suitable for a USB stick\n"))
   (display (_ "\
-  - 'init', initialize a root file system to run GNU.\n"))
+   init             initialize a root file system to run GNU.\n"))
 
   (show-build-options-help)
   (display (_ "