Add warning to RawValue docs regarding untagged enums

[d4h0]

Hi,

This adds a warning to the docs of RawValue for this bug (deserializing to an untagged enum with RawValue always fails).

Because there is basically no usable information if deserializing to an untagged enum failes, a note like this would be useful to Rust beginners like me (it took me more than one hour to find out what the problem was).