CDRIVER-4363 document `mongoc_bulkwrite_t` (#1602)

* document `mongoc_bulkwrite_t`

* rename `set_hint_opt` to `set_bson_value_opt`

To reuse for the `comment` option

* use `bson_value_t` for `comment`, not `bson_t`

Matches the spec, and existing C driver representation of `comment` for other functions.

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Co-authored-by: Roberto C. Sánchez <roberto@connexer.com>