changefeedccl: better doc for changefeed.min_highwater_advance

[wenyihu6]

This patch updates the documentation for changefeed.min_highwater_advance since the name and the description could be misleading.

Epic: none Release note: none

[blathers-crl[bot]]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{:owl: Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[wenyihu6]

I would want to change the cluster setting name as well, but I'm not sure if this counts as a breaking change. Wdyt?