Package extensions: fixed a link in docs (#16040)

* Moved link to the right place in the docs

* Fixed a few minor issues in extensions docs

Fixed a typo, added a subsubsection for better
navigation, reworded "modules in Python" as
"Python packages"

lib/spack/docs/basic_usage.rst

@@ -1234,6 +1234,8 @@ add a version specifier to the spec:
 Notice that the package versions that provide insufficient MPI
 versions are now filtered out.
 
+.. _extensions:
+
 ---------------------------
 Extensions & Python support
 ---------------------------
@@ -1241,8 +1243,7 @@ Extensions & Python support
 Spack's installation model assumes that each package will live in its
 own install prefix.  However, certain packages are typically installed
 *within* the directory hierarchy of other packages.  For example,
-modules in interpreted languages like `Python
-<https://www.python.org>`_ are typically installed in the
+`Python <https://www.python.org>`_ packages are typically installed in the
 ``$prefix/lib/python-2.7/site-packages`` directory.
 
 Spack has support for this type of installation as well.  In Spack,

lib/spack/docs/module_file_support.rst

@@ -165,8 +165,6 @@ used ``gcc``.  You could therefore just type:
 
 To identify just the one built with the Intel compiler.
 
-.. _extensions:
-
 .. _cmd-spack-module-loads:
 
 ^^^^^^^^^^^^^^^^^^^^^^^^^^

lib/spack/docs/packaging_guide.rst

@@ -2197,7 +2197,7 @@ property to ``True``, e.g.:
        extendable = True
        ...
 
-To make a package into an extension, simply add simply add an
+To make a package into an extension, simply add an
 ``extends`` call in the package definition, and pass it the name of an
 extendable package:
 
@@ -2212,6 +2212,10 @@ Now, the ``py-numpy`` package can be used as an argument to ``spack
 activate``.  When it is activated, all the files in its prefix will be
 symbolically linked into the prefix of the python package.
 
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Adding additional constraints
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 Some packages produce a Python extension, but are only compatible with
 Python 3, or with Python 2.  In those cases, a ``depends_on()``
 declaration should be made in addition to the ``extends()``
@@ -2231,8 +2235,7 @@ variant(s) are selected.  This may be accomplished with conditional
 .. code-block:: python
 
    class FooLib(Package):
-       variant('python', default=True, description= \
-           'Build the Python extension Module')
+       variant('python', default=True, description='Build the Python extension Module')
        extends('python', when='+python')
        ...