[3.14] gh-138577: Mention Unix-specific limitations of getpass.getpass(echo_char=...) (GH-138677) (#138696)

Co-authored-by: yagggi <[email protected]>

Author: miss-islington

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.