Issue #17386: Update Doc/README.txt to list all targets

supported by Doc/Makefile, as well as a bit of modernization,
cleanup, and notes about the difference between Unix and Windows.

Doc/README.txt

@@ -5,7 +5,7 @@ This directory contains the reStructuredText (reST) sources to the Python
 documentation.  You don't need to build them yourself, prebuilt versions are
 available at <https://docs.python.org/3.4/download.html>.
 
-Documentation on the authoring Python documentation, including information about
+Documentation on authoring Python documentation, including information about
 both style and markup, is available in the "Documenting Python" chapter of the
 developers guide <http://docs.python.org/devguide/documenting.html>.
 
@@ -26,8 +26,7 @@ Sphinx, you can just run ::
 
    make html
 
-to build the HTML output files.  To view the generated HTML, point your favorite
-browser at the top-level index `build/html/index.html` after running "make".
+to build the HTML output files.
 
 On Windows, we try to emulate the Makefile as closely as possible with a
 ``make.bat`` file.
@@ -35,18 +34,29 @@ On Windows, we try to emulate the Makefile as closely as possible with a
 To use a Python interpreter that's not called ``python``, use the standard
 way to set Makefile variables, using e.g. ::
 
-   make html PYTHON=/usr/bin/python2.5
+   make html PYTHON=python3
+
+On Windows, set the PYTHON environment variable instead.
+
+To use a specific sphinx-build (something other than ``sphinx-build``), set
+the SPHINXBUILD variable.
 
 Available make targets are:
 
+ * "clean", which removes all build files.
+
  * "html", which builds standalone HTML files for offline viewing.
 
+ * "htmlview", which re-uses the "html" builder, but then opens the main page
+   in your default web browser.
+
  * "htmlhelp", which builds HTML files and a HTML Help project file usable to
    convert them into a single Compiled HTML (.chm) file -- these are popular
    under Microsoft Windows, but very handy on every platform.
 
-   To create the CHM file, you need to run the Microsoft HTML Help Workshop over
-   the generated project (.hhp) file.
+   To create the CHM file, you need to run the Microsoft HTML Help Workshop
+   over the generated project (.hhp) file.  The make.bat script does this for
+   you on Windows.
 
  * "latex", which builds LaTeX source files as input to "pdflatex" to produce
    PDF documents.
@@ -75,6 +85,13 @@ Available make targets are:
  * "suspicious", which checks the parsed markup for text that looks like
    malformed and thus unconverted reST.
 
+ * "check", which checks for frequent markup errors.
+
+ * "serve", which serves the build/html directory on port 8000.
+
+ * "dist", (Unix only) which creates distributable archives of HTML, text,
+   PDF, and EPUB builds.
+
 
 Without make
 ------------