780 Document incompatibility in format-number etc - #925

I added changes to the coercion rules that allow an untypedAtomic value to be supplied where the required type is (for example) union(string, QName). Without this, we would have an additional incompatibility from the format-number() change, because it would no longer be allowed to supply an untyped node as the third argument. (This is most likely to happen where the value is actually a text node resulting from evaluation of an xsl:variable declaration).

I also made the change suggested in issue #911 to change the table caption in the coercion rules from "type promotion" to "implicit casting".

The CG agreed to merge this PR without discussion at meeting 060

Note: the original change to allow an xs:QName argument to functions such function-available and element-available was made in the initial editor's draft accepted by the CG as its baseline. I have arbitrarily linked test cases etc to this PR as an identifier for the change.