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

In bf8bbe9, `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.
(cherry picked from commit 074f3b2)

Co-authored-by: yagggi <fakepoet0101@gmail.com>

Author: yagggi

Doc/library/getpass.rst

@@ -39,6 +39,14 @@ The :mod:`getpass` module provides two functions:
       If you call getpass from within IDLE, the input may be done in the
       terminal you launched IDLE from rather than the idle window itself.
 
+   .. note::
+      On Unix systems, when *echo_char* is set, the terminal will be
+      configured to operate in
+      :manpage:`noncanonical mode <termios(3)#Canonical_and_noncanonical_mode>`.
+      In particular, this means that line editing shortcuts such as
+      :kbd:`Ctrl+U` will not work and may insert unexpected characters into
+      the input.
+
    .. versionchanged:: 3.14
       Added the *echo_char* parameter for keyboard feedback.