Add Supported syntax Values to Expression Field Documentation

[SalemBajjali]

The syntax field in the documentation is currently defined as:

The syntax used to describe the variation. The value should be one of the supported syntaxes.

However, the list of supported syntax values isn’t included in the documentation. At the moment, the only way to find them is by looking through the source code. To improve clarity and ease of use, it would be helpful if the documentation explicitly listed the supported values. Based on what’s in the code, they appear to be:

["hgvs.c", "hgvs.p", "hgvs.g", "hgvs.m", "hgvs.n", "hgvs.r", "iscn", "gnomad", "spdi"]

[korikuzma]

Personally, I don't love hard coding the values twice (easy to forget to update in both places). I wonder if we can have MSP generate a new sentence to list the enums?