sql: document know limitation about inverted indexes on primary key columns

[jordanlewis]

Jordan Lewis (jordanlewis) commented:

Starting in 22.2, there will be a datatype in SQL (text) that's indexable both with an ordinary index and with an inverted index (using the gin_trgm_ops trigram inverted index flag). We're adding a known limitation in https://github.com/cockroachdb/cockroach/pull/84412 that prevents inverted indexing a column that's also part of the primary key.

It would be great to document this in general, and especially in the case of trigrams.

Jira Issue: DOC-5022