bug#24221: 25.1.50; binding anonymous functions to keys cause help buffer navigation errors

[Drew Adams]

> > C-h k C-c l
> > And a help buffer for something else:
> > C-h k a
> >
> > Then, in the help buffer, go to the bottom, and try to select the
> > "[back]" link. I get an error: "user-error: You didn’t specify a
> > function symbol".

This is apparently a regression from Emacs 24.5.

I see the same regression in this older pre-Emacs 25 build:

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2015-05-29 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/mingw32 --host=i686-pc-mingw32
 --enable-checking=yes,glyphs'

So it seems to have been introduced on or before 2015-05-29.

And I do not see the regression in this older pre-25 build:

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2015-04-05 on LEG570
Repository revision: 16eec6fc55dcc05d1d819f18998e84a9580b2521
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --host=i686-pc-mingw32 --enable-checking=yes,glyphs'

So it seems to have been introduced after 2015-04-05, i.e., between
2015-04-05 and 2015-05-29.

> I don't know what the best fix would be - fix the xref setup in
> `describe-key' or teach `describe-function' to do something useful with
> a lambda expression.

Remove the regression.  Maybe restore some of the code from before
it was introduced.