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

[Eli Zaretskii]

> Date: Wed, 10 Apr 2019 15:13:31 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
> 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.

Thanks, fixed.

> 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.

Done.