bug#53328: 27.2; Doc-string line incorrectly filled (off-by-one bug)

[Drew Adams]

emacs -Q

(defface foo
  '((t :background "Green"))
  "Face used to @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
xxxxx xxxxxxxxxxx xxxxxxxxxxxx xxxxxxxxxxxxx  xxxxxxx  xxxxxx yyyyyyy."
  :group 'faces)

Put point at the beginning of the second doc-string line.

`M-q'

That second line is filled by inserting a newline char before `yyyyyyy'.
The line should not be split - it is 70 chars, which is allowed by
convention.

The problem seems to be an off-by-one bug.  The double-quote char ending
the string is perhaps being counted, as if it were part of the doc
string.

In GNU Emacs 27.2 (build 1, x86_64-w64-mingw32)
 of 2021-03-26 built on CIRROCUMULUS
Repository revision: deef5efafb70f4b171265b896505b92b6eef24e6
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.19043
System Description: Microsoft Windows 10 Pro (v10.0.2009.19043.1466)