feat(docs): #1296 add note about homeless-shelter

- Add note about homeless-shelter in the python extension documentation
  to make sure that the user knows that we delete this directory and
  change the `HOME` variable to assure purity of builds without
  sandboxing.

Signed-off-by: Robin Quintero <rohaquinlop301@gmail.com>

docs/src/api/extensions/python.md

@@ -24,6 +24,18 @@ Types:
         For more information see [here](https://github.com/nix-community/poetry2nix/blob/master/docs/edgecases.md).
         Defaults to `(self: super: {})`.
 
+        Note:
+
+        - By default we override every python package deleting the
+          `homeless-shelter` directory and changing the `HOME` variable,
+          we make this to assure purity of builds without sandboxing. If
+          any build attribute is being override, please have this in mind
+          if any error related to `homeless-shelter` start to appear.
+        - We delete this directory in the `preUnpack` phase because this
+          is the first phase that will be executed and with that we make
+          sure that the `HOME` will be used during the execution of the
+          next phases.
+
 Example:
 
 === "main.nix"
@@ -43,6 +55,12 @@ Example:
       overrides = self: super: {
         pygments = super.pygments.overridePythonAttrs (
           old: {
+            preUnpack =
+              ''
+                export HOME=$(mktemp -d)
+                rm -rf /homeless-shelter
+              ''
+              + (old.preUnpack or "");
             buildInputs = [super.setuptools];
           }
         );