bug#57278: 26.3; Doc of `delete-indentation'

[Drew Adams]

The doc string and the description in (elisp) `User-Level Deletion' are
insufficiently oriented to end usage.  They should say clearly that a
prefix argument joins this line to the next one.

The doc string talks about "argument".  It should say "a prefix
argument".  The manual says nothing about the prefix argument.  It talks
only about argument JOIN-FOLLOWING-P.

Please write the descriptions first of all with interactive use in mind,
speaking of the prefix arg.  Then say that when called from Lisp the
argument corresponds to the prefix argument when used interactively.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.19044
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''