bug#35222: 26.1; `read-command' documentation

[Drew Adams]

1. The doc string does not say what happens if DEFAULT is not provided
   (so `nil') and the user enters empty input.  And what happens is not
   obvious.  Please add that information to the doc string.

2. What does happen?  Well, it's documented in (elisp) `High-Level
   Completion'.  But please add information to that Info node that helps
   users understand a return value that is an empty-named interned
   symbol, which has the print and read syntax `##'.  It's pretty
   unusual for the default behavior of a read command to return such an
   odd beast.  (The default behavior is the case when arg DEFAULT is
   absent, hence `nil'.)  And please provide a cross-reference to node
   `Symbol Type', which explains this syntax.

   User code that tests the result of `read-command' typically needs to
   deal with the default DEFAULT case, hence with the possibility of the
   function returning an uninterned symbol.  Users need to be aware of
   this possibility - it's a gotcha.


In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
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''