doc: add docs for bson_context_t.
doc: add bson_error_t docs.
doc: add bson_iter_t docs.
doc: add bson_json_reader_t doc.
oid: allow length of 25 if str[24] is \0
doc: add bson_oid_t page.
doc: add docs for bson_reader_t.
doc: add docs on bson_string_t.