Floating point rounding modes: we should restrict them slightly

[Walter Bright]

Don wrote:
> PROPOSAL:
> Change the spec by adding the line to float.html:
> "If the floating-point rounding mode is changed within a function, it 
> must be restored before the function exits. If this rule is violated 
> (for example, by the use of inline asm), the rounding mode used for 
> subsequent calculations is undefined."


I added it into float.html for D2.