tools: sphinx-build-wrapper: improve its help message

Besides the parameters that are passed via command line arguments,
the wrapper's behavior is affected by several environment variables.

Document that. While here, use __doc__ for its description.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <e0ccee75f7e7fb499e0f59d5b84469f4b6a21627.1769500383.git.mchehab+huawei@kernel.org>

Author: mchehab

tools/docs/sphinx-build-wrapper

@@ -814,20 +814,42 @@ def jobs_type(value):
     except ValueError:
         raise argparse.ArgumentTypeError(f"Must be 'auto' or positive integer, got {value}")  # pylint: disable=W0707
 
+EPILOG="""
+Besides the command line arguments, several environment variables affect its
+default behavior, meant to be used when called via Kernel Makefile:
+
+- KERNELVERSION:  Kernel major version
+- KERNELRELEASE:  Kernel release
+- KBUILD_VERBOSE: Contains the value of "make V=[0|1] variable.
+                  When V=0 (KBUILD_VERBOSE=0), sets verbose level to "-q".
+- SPHINXBUILD:    Documentation build tool (default: "sphinx-build").
+- SPHINXOPTS:     Extra options pased to SPHINXBUILD
+                  (default: "-j auto" and "-q" if KBUILD_VERBOSE=0).
+                  The "-v" flag can be used to increase verbosity.
+                  If V=0, the first "-v" will drop "-q".
+- PYTHON3:        Python command to run SPHINXBUILD
+- PDFLATEX:       LaTeX PDF engine. (default: "xelatex")
+- LATEXOPTS:      Optional set of command line arguments to the LaTeX engine
+- srctree:        Location of the Kernel root directory (default: ".").
+
+"""
+
 def main():
     """
     Main function. The only mandatory argument is the target. If not
     specified, the other arguments will use default values if not
     specified at os.environ.
     """
-    parser = argparse.ArgumentParser(description="Kernel documentation builder")
+    parser = argparse.ArgumentParser(formatter_class=argparse.RawTextHelpFormatter,
+                                     description=__doc__,
+                                     epilog=EPILOG)
 
     parser.add_argument("target", choices=list(TARGETS.keys()),
                         help="Documentation target to build")
     parser.add_argument("--sphinxdirs", nargs="+",
                         help="Specific directories to build")
     parser.add_argument("--builddir", default="output",
-                        help="Sphinx configuration file")
+                        help="Sphinx configuration file (default: %(default)s)")
 
     parser.add_argument("--theme", help="Sphinx theme to use")
 
@@ -843,7 +865,7 @@ def main():
                         help="place build in verbose mode")
 
     parser.add_argument('-j', '--jobs', type=jobs_type,
-                        help="Sets number of jobs to use with sphinx-build")
+                        help="Sets number of jobs to use with sphinx-build(default: auto)")
 
     parser.add_argument('-i', '--interactive', action='store_true',
                         help="Change latex default to run in interactive mode")