Add doc comments to icu_uniset

[iainireland]

This resolves #1030.

A few notes:

1. I removed UnicodeSetSpanCondition, which appears to be unused, instead of documenting it.
2. I stole most of the wording for individual properties from ICU4C / various Unicode TRs
3. I think there's a case to be made for removing some of the properties (in particular, there are a couple that are deprecated). We can do that in a separate issue.
4. I did not add individual doc comments for scripts or keys.

[echeran]

I added suggested changes regarding the Error type variants, since we still seem to use displaydoc in other components similarly, but instead just add doc strings on top. Otherwise, LGTM.

[echeran]

LGTM!