__netbox: add manpage warning for "big version jumps"

As getting myself into trouble while upgrading from 2.10 to 3.1 cause of
an migration break which allowing upgrades only from 2.11, I've add this
warning or notice to be a bit more aware of this.

type/__netbox/gencode-remote

@@ -51,7 +51,9 @@ fi
 # Install python dependencies.
 #  avoid gunicorn, because it will be done in an other type
 grep -v "^gunicorn==" "\$tmpdir/$src/requirements.txt" \
-    | xargs /opt/netbox/venv/bin/pip3 install -q
+    > "\$tmpdir/$src/requirements.txt.new"
+/opt/netbox/venv/bin/pip3 install -q -r "\$tmpdir/$src/requirements.txt.new"
+
 EOF
 
     if [ -f "$__object/parameter/ldap-server" ]; then

type/__netbox/man.rst

@@ -31,6 +31,12 @@ version
     on GitHub at the NetBox project page under
     "`Releases <https://github.com/netbox-community/netbox/releases>`_".
 
+    Too big version jumps can break the NetBox migration path. It's good
+    practise to don't skip major versions and common that you must upgrade to
+    the latest minor inside the current major version till you can upgrade to
+    the next major version. Diffrent version steps must be done manually as
+    this type only upgrades to the given version directly.
+
 database
     PostgreSQL database name.