bug#35651: 26.2; doc of `delimit-columns-*' commands

[Drew Adams]

The doc strings are woefully inadequate.  "Prettify all columns..."  is
unclear.  The behavior - what the commands do - is not described at all.

And there is nothing in any manual about these commands either.  Users
are forced to consult the code to have any idea of what this "delimit
columns" stuff is all about.

Please do the Emacs thing for these commands: document them for users.

In GNU Emacs 26.2 (build 1, x86_64-w64-mingw32)
 of 2019-04-13
Repository revision: fd1b34bfba8f3f6298df47c8e10b61530426f749
Windowing system distributor `Microsoft Corp.', version 10.0.17134
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''