Remove ASN1_STRING_TABLE_{add,cleanup}() documentation
The unused ASN1_STRING_TABLE extensibility API will be removed in the next
major bump and the table itself will become immutable. Lightly adjust the
remaining text. In particular, update the RFC reference, stop talking about
defaults when nothing can be changed anymore, do not mention useless flags
that you will no longer be able to set and move the description of the only
remaining flag after the description of ASN1_STRING_TABLE_get().
The file will be renamed in a second step.