bug#51586: 27.2; Customize inserts trailing-space empty lines in doc string

[Drew Adams]

emacs -Q
Eval this code, then `M-x customize-option foo'

(defcustom foo 42
  "Some doc.
Several possibilities:

* `xxx' - yyyyyyyyy yyyyyyyyyyyy

* `yyy' - zzzzzzzzzzzzzz zzzzzzzzz

* 42 - aaaaaa aaaaaaaaa
  Some more description of this.
  And some more.

  And yet more
  explanation of it.

* `qqq' - rrrrrrrrr rrrrrrrrrr"
  :group 'emacs :type 'sexp)

In the Customize buffer, The empty lines in the doc string get filled
with SPC chars up to the indentation.  That is, trailing whitespace
chars are inserted.  If you turn on highlighting of trailing whitespace
you can see this.  Shouldn't happen - the lines should remain empty.


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 10.0.19042
System Description: Microsoft Windows 10 Pro (v10.0.2009.19042.1288)