(zilch lang ninja): document

Change-Id: I6a6a6964dcc713504ec57f40981a601696a573af

cli/zilch-ninja.scm

@@ -5,35 +5,13 @@
   (foreign-lambda* int ()
     "cpu_set_t set; sched_getaffinity(0, sizeof(set), &set); C_return(CPU_COUNT(&set));"))
 
+(foreign-declare "#include \"man_ninja.h\"")
+(define man-page (foreign-value "man_ninja" nonnull-c-string))
+
 (define (print-help msg)
   (when msg
     (write-string (string-append msg "\n\n") (current-error-port)))
-  (write-string "Usage: zilch-cli-ninja [OPTION] SUBCOMMAND ...
-Processes a Ninja build based on the configuration in the passed-in
-config file (or zilch.scm, if not set), and reproducibly builds either
-to final build, or to specific Ninja targets.
-
-Supported commands:
-  build [TARGET]          Build the full drv, or specific targets from
-                          its Ninja file.
-  source [DIR]            Extract the source of the derivation to DIR
-                          (or src, if unspecified), ready for Zilch.
-  diff                    Print the difference between the original and
-                          modified sources as found in either `src' or
-                          the directory selected by --source.
-
-Arguments:
-  -h, --help              Print this help message.
-  -f, --config-file PATH  Path to the Zilch config file.
-  -s, --source DIR        Override the input source for builds. Doesn't
-                          reconfigure the build, so changes to the
-                          build system will not apply.
-  -j, --max-jobs COUNT    The maximum amount of builds to run. Defaults
-                          to the amount of cores.
-  -v, --verbose           Increase the verbosity configured in the Nix
-                          daemon.
-  -L, --print-build-logs  Print derivation logs as they come in.
-" (current-error-port))
+  (write-string man-page (current-error-port))
   (exit (or (not msg) 1)))
 
 (define-values (options args)