docs: warn against @big_str not obeying rounding mode or precision

[nsajko]

One of the big pitfalls when using BigFloat.

[rfourquet]

The intent is good, but I feel the wording must be improved. Indeed it says

As a macro, @big_str can't obey the global rounding mode or precision settings.
I.e., it is not affected by setprecision

But it is affected by the global precision setting:

julia> precision(big"1.2")
256

julia> setprecision(10)
10

julia> precision(big"1.2")
10

I think what you meant is that due to the way macros are evaluated, the global precision which is used is the one at play when the code is loaded rather than evaluated.
I.e. in a new session

julia> setprecision(20) do
           precision(big"1.2")
       end
256

[nsajko]

ping