bug#6581: 24.0.50; `equal-including-properties' uses `eq' to compare property values

[Drew Adams]

> I've just hit this bug today.  Half an hour wasted.

There certainly is *at least* a doc bug.  Users of such
a function at least need to be told how the text property
values are compared.  That's a minimum for the doc of any
equality predicate: say just what equality means in this
case.

Both the doc string and the Elisp manual (node `Equality
Predicates') are deficient.

In addition, a reasonable enhancement might be (now) to
accept an optional comparison (equality) predicate,
especially to be able to work around the designed behavior.

But I'm guessing that the default behavior could/should
not be changed at this point, because existing code
could break.

Just call out the *actual behavior*, clearly and completely.
Even LOUDLY, as this is truly a gotcha.  (And one needs to
check C code to see what the function really does.)

This predicate was added after Emacs 20 (after 21 perhaps?).
It's a bit surprising that no one has thought of completing
the doc - at least.