profiles: improve Link message encoding documentation

[jhalliday]

Clarify appropriate representation of invalid Links for better compatibility with existing codecs.

[jhalliday]

@open-telemetry/profiling-maintainers per #8349 #issuecomment-4352927214 experience has shown that existing span id / trace id codecs can be strict about type length, i.e. don't play well with the profiles alpha spec definition of invalidity as 'minimum message bytes on the wire'
This doc change clarifies that the ProfilesDictionary.link_table[0] element should be encoded such as to maximise compatibility with the existing ecosystem at the cost of a small increase in message payload size.
As it still permits the previous encoding this is not considered a breaking change and could be merged immediately, unlike e.g. #786