Add more instruction descriptions

[sampsyo]

Many instructions are missing short English-language synopses. Presumably, this is because UDB doesn't have descriptions of them (but we should confirm that we're not just missing them during parsing).

Let's try to fill in as many of these as we can, by either:

• opening PRs on the UDB repository to add the missing data
• writing our own YAML file or whatever for synopses that we then use as a supplement to UDB
• find some other pre-existing data source we can reuse