Document `isapprox`

[dpsanders]

julia> isapprox(1..2, 3..4)
ERROR: ArgumentError: `==` is purposely not supported for intervals. See instead `isequal_interval`
Stacktrace:
 [1] ==(x::Interval{Float64}, y::Interval{Float64})
   @ IntervalArithmetic ~/Dropbox/packages/IntervalArithmetic/src/intervals/real_interface.jl:86
 [2] #isapprox#809
   @ ./floatfuncs.jl:308 [inlined]
 [3] isapprox(x::Interval{Float64}, y::Interval{Float64})
   @ Base ./floatfuncs.jl:304
 [4] top-level scope
   @ REPL[3]:1

This should either work or fail with a sensible error message.

[OlivierHnt]

There is a section in the documentation Comparisons and set operations whose first sentence reads "All comparisons and set operations for Real have been purposely disallowed to prevent silent errors".

We should explicitly mention isapprox there.

[dpsanders]

Sorry I meant that isapprox has not been explicitly disallowed - we haven't defined it (to error) for intervals. It's falling back to a generic definition in base as far as I can see.

[OlivierHnt]

Actually it was disallowed on purpose. We currently just rely on a fallback error.
This is the case for several other functions, eg symdiff or intersect, etc. (see intersect(interval(1, 2), interval(1, 2))).

That being said, I am not opposed to customise the error message eventually.