Document Ansible's Python interpreter discovery some more

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/570

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/641

docs/prerequisites.md

@@ -10,7 +10,7 @@ This playbook doesn't support running on ARM (see [this issue](https://github.co
 
 - `root` access to your server (or a user capable of elevating to `root` via `sudo`).
 
-- [Python](https://www.python.org/) being installed on the server. Most distributions install Python by default, but some don't (e.g. Ubuntu 18.04) and require manual installation (something like `apt-get install python`).
+- [Python](https://www.python.org/) being installed on the server. Most distributions install Python by default, but some don't (e.g. Ubuntu 18.04) and require manual installation (something like `apt-get install python3`). On some distros, Ansible may incorrectly [detect the Python version](https://docs.ansible.com/ansible/latest/reference_appendices/interpreter_discovery.html) (2 vs 3) and you may need to explicitly specify the interpreter path in `inventory/hosts` during installation (e.g. `ansible_python_interpreter=/usr/bin/python3`)
 
 - A `cron`-like tool installed on the server such as `cron` or `anacron` to automatically schedule the Let's Encrypt SSL certificates's renewal. *This can be ignored if you use your own SSL certificates.*
 

examples/hosts

@@ -10,6 +10,9 @@
 #
 # If you're running this Ansible playbook on the same server as the one you're installing to,
 # consider adding an additional `ansible_connection=local` argument below.
+#
+# Ansible may fail to discover which Python interpreter to use on the host for some distros (like Ubuntu 20.04).
+# You may sometimes need to explicitly add `ansible_python_interpreter=/usr/bin/python3` to lines below.
 
 [matrix_servers]
 matrix.<your-domain> ansible_host=<your-server's external IP address> ansible_ssh_user=root