Improve minpoolsize docs:

http://api.mongodb.org/c/current/mongoc_uri_t.html
http://api.mongodb.org/c/current/mongoc_client_pool_min_size.html

Don't close any clients pushed into pool if min size is 0.

Introduced in https://github.com/mongodb/mongo-c-driver/pull/171, specifically:

https://github.com/mschoenlaub/mongo-c-driver/commit/4d0ff8c6c807ef998adafc2b21c50e4f4983fc34#diff-570835aa57126f0dd8f862fa975ff0c3R204

This appears to date to release 1.1.2. I marked this "affects version 1.1.11" since that was the first version tracked in Jira.