Add footnote to lang strings in SIP spec

[lucasotodegraeve]

I changed the the maximum cardinality of lang strings to 1 for the descriptive metadata part of the SIP spec.

I interpret the notation of the cardinality 1..1 on a lang string as:

<dcterms:title xml:lang="nl">mijn titel</dcterms:title>
<dcterms:title xml:lang="fr">mon titre</dcterms:title>

Where "the same semantic thing" might appear multiple times in different languages.

However, having 0..* or 1..* doesn't make much sense for lang string:

<dcterms:alternative xml:lang="nl">mijn alternative titel</dcterms:alternative>
<dcterms:alternative xml:lang="fr">mon titre alternatif</dcterms:alternative>

<dcterms:alternative xml:lang="nl">een tweede alternative titel</dcterms:alternative>
<dcterms:alternative xml:lang="fr">un deuxième titre alternatif</dcterms:alternative>

There is no way to know which strings belong together semantically. Do you agree? Is the current notation for cardinality good as is? In the datamodels the notation is slightly different.

[lucasotodegraeve]

As discussed, I've updated the cardinalities to be either 1..* or 0..* and include a reference to a footnote.