emacs -Q
C-h f kill-line RET
Follow the link to simple.el
Look for the end of the docstring for kill-line and note that the
ending " gets font-lock-warning-face.

Alternatively, copy this into the *scratch* buffer:
(defun foo ()
  "Some docstring.

(Note that it ends in the
next line.)"
  (ignore))

Again, the ending double quote gets font-lock-warning-face.

It looks like any docstring that ends with a ) that expands for more
than one line gets the ending " character fontified with
font-lock-warning-face.


In GNU Emacs 28.0.50 (build 18, x86_64-pc-linux-gnu, GTK+ Version 3.22.30,
cairo version 1.15.10)
 of 2020-11-06 built on tbb-desktop
Repository revision: aea936562b033899f0b7160cc492bbea1006863b
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Ubuntu 18.04.5 LTS

Configured features:
XPM JPEG TIFF GIF PNG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY
LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 XDBE XIM MODULES THREADS PDUMPER

Important settings:
  value of $LC_MONETARY: es_AR.UTF-8
  value of $LC_NUMERIC: es_AR.UTF-8
  value of $LC_TIME: es_AR.UTF-8
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec epa derived epg epg-config gnus-util rmail
rmail-loaddefs auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json map text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils pp wid-edit
descr-text help-mode easymenu cl-loaddefs cl-lib tooltip eldoc electric
uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice button
loaddefs faces cus-face macroexp files window text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads dbusbind inotify
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 56032 12078)
 (symbols 48 19403 1)
 (strings 32 76733 2131)
 (string-bytes 1 1826970)
 (vectors 16 12632)
 (vector-slots 8 568845 14525)
 (floats 8 26 41)
 (intervals 56 303 0)
 (buffers 992 13))