Add and update pynsist template comments
diff --git a/windows-installer/assets/template.nsi b/windows-installer/assets/template.nsi
index 64bceb06551..566e1b004cc 100644
--- a/windows-installer/assets/template.nsi
+++ b/windows-installer/assets/template.nsi
@@ -1,7 +1,7 @@
-; This NSIS template is based on the built-in one in pynsist 2.6.
+; This NSIS template is based on the built-in one in pynsist 2.7.
 ; Added lines are enclosed within "CERTBOT CUSTOM BEGIN/END" comments.
 ; If pynsist is upgraded, this template must be updated if necessary using the new built-in one.
-; Original file can be found here: https://github.com/takluyver/pynsist/blob/2.6/nsist/pyapp.nsi
+; Original file can be found here: https://github.com/takluyver/pynsist/blob/2.7/nsist/pyapp.nsi
 
 !define PRODUCT_NAME "[[ib.appname]]"
 !define PRODUCT_VERSION "[[ib.version]]"
diff --git a/windows-installer/setup.py b/windows-installer/setup.py
index 01d75a99bd8..d5b4d2bde12 100644
--- a/windows-installer/setup.py
+++ b/windows-installer/setup.py
@@ -29,6 +29,12 @@
     packages=find_packages(),
     include_package_data=True,
     install_requires=[
+        # pynsist is pinned to an exact version so we can update
+        # assets/template.nsi as needed. The file is based on the default
+        # pynsist NSIS template and pynsist's documentation warns that custom
+        # templates may need to be updated for them to work with new versions
+        # of pynsist. See
+        # https://pynsist.readthedocs.io/en/latest/cfgfile.html#build-section.
         'pynsist==2.7'
     ],
     entry_points={

This was an afterthought from https://github.com/certbot/certbot/pull/8749. I just wanted to document things a bit more.