The current documentation for read_concern::acknowledge_string() states that the parameter "rc_string" needs to be 'One of "local", "majority", "linearizable", or ""'. For better compliance with the Read and Write Concern spec (section "Unknown Levels and Additional Options for String Based ReadConcerns"), we should clarify that any read concern string can be passed in.

In addition, since our code/documentation assumes that "local" translates to MONGOC_READ_CONCERN_LEVEL_LOCAL / etc, we should consider adding static assertions to this effect, to reduce the risk of regressions introduced due to possible future changes in the C driver.