doc: add warning for self update breakages

Relates-to: #4040

README.md

@@ -92,6 +92,9 @@ python install-poetry.py --git https://github.com/python-poetry/poetry.git@maste
 
 
 Updating poetry to the latest stable version is as simple as calling the `self update` command.
 Updating poetry to the latest stable version is as simple as calling the `self update` command.
 
 
+**Warning**: Poetry versions installed using the now deprecated `get-poetry.py` installer will not be able to use this
+command to update to 1.2 releases or later. Migrate to using the `install-poetry.py` installer or `pipx`.
+
 ```bash
 ```bash
 poetry self update
 poetry self update
 ```
 ```

docs/docs/index.md

@@ -133,6 +133,11 @@ pip install --user poetry
 
 
 Updating Poetry to the latest stable version is as simple as calling the `self update` command.
 Updating Poetry to the latest stable version is as simple as calling the `self update` command.
 
 
+!!!warning
+
+    Poetry versions installed using the now deprecated `get-poetry.py` installer will not be able to use this
+    command to update to 1.2 releases or later. Migrate to using the `install-poetry.py` installer or `pipx`.
+
 ```bash
 ```bash
 poetry self update
 poetry self update
 ```
 ```