gh-138577: Mention Unix-specific limitations of `getpass.getpass(echo_char=...)` (#138677)

In bf8bbe9a813dd9fc2dd14be06df172b7d26ca1af, `getpass.getpass` gained
the ability to provide keyboard feedback through `echo_char`.

On Unix, line editing shortcuts such as Ctrl+U were previously handled
as the terminal operates in canonical mode (see termios(3)). However,
since keyboard feedback requires to switch to noncanonical mode, this
now results in an inconsistency when `getpass.getpass` uses `echo_char`
as those shortcuts are no more supported. This limitation is specific
to Unix and does not affect Windows users where line editing shortcuts
were never supported.