Feature Request: clarify flag documentation for --db_charset

[mcrauwel]

Feature Description

the --db_charset flag is currently documented as follows:

--db_charset string                                                Character set used for this tablet. (default "utf8mb4")

While this flag allows for a collation to be configured as well.
Suggestion to change the description as follows:

Character/Collation set used for this tablet, make sure this charset/collation is supported by the lowest MySQL version in the cluster. (default "utf8mb4")

Use Case(s)

In the process of migrating from MySQL 5.7 to 8.0.

utf8mb4 has a different default collation in both 5.7 and 8.0 specifying the 5.7 default collation utf8mb4_general_ci in this flag will allow there replication from 8.0 back to 5.7 to work

if we don't specify a collation supported by the lowest of MySQL versions, replication will break.