StringCharset BugPattern summary is not helpful

The summary for the StringCharset ([documentation](https://errorprone.info/bugpattern/StringCharset), [code](https://github.com/google/error-prone/blob/4ce508bfa4add43094274c4a94a140e238e2e849/core/src/main/java/com/google/errorprone/bugpatterns/StringCharset.java#L44)) BugPattern doesn't describe the problem well, is the same as the bugpattern's name. When a bug is reported, the users see the summary of the bug, and this summary is not really helpful deciding what to do to correct the code. To understand what the user should do, he/she needs to check out the documentation of the bugpattern.

See [EqualsNaN](https://errorprone.info/bugpattern/EqualsNaN) bugpattern as a good example for having a great summary.

Error Prone version: 2.26.1 (and current master)