Document the limitations of the pip install --prefix argument, and cross-reference the --python flag, which can be harder to find due to it being a pip level argument

Author: pelson

src/pip/_internal/commands/install.py

@@ -156,7 +156,12 @@ def add_options(self) -> None:
             default=None,
             help=(
                 "Installation prefix where lib, bin and other top-level "
-                "folders are placed"
+                "folders are placed. \n"
+                "Note that the resulting installation may "
+                "contain scripts and other resources which reference the "
+                "Python interpreter of pip, and not that of ``--prefix``. \n"
+                "See also the ``pip --python`` option if the intention is to "
+                "install packages into another (possibly pip-free) environment."
             ),
         )