bug#13823: 24.3.50; Elisp manual description of property `pure'

[Drew Adams]

> go straight to the point.  E.g. ask "Could we have it
> be properly documented, please?".

Oh, that's just what I did, in bug #13052:

>> How about documenting this `pure' property?

and later in that same thread:

>> In any case, this property should be documented.

Another user agreed and added that "We can be happy that [Emacs Dev does not
just tell users] `Do not use this'."

To which, ironically, the response was in fact to "document" it by just telling
users "Do not set it."

Bug #13823 is an attempt to get beyond that reaction constructively.

You asked me to come up with a description of what it does.  I proposed some
text based on my understanding, inviting correction.  Dunno whether you even
read it.

Your response was to tell me I should not assume you know more about this than
I, and that I should anyway just ask that it be properly documented.  So one
more time:

"Could we have it be properly documented, please?"

It is the only standard symbol property listed in (elisp) `Standard Properties'
about which we say virtually nothing and for which there is no cross reference
to a section that covers it.