Improve messsages in the bundled signatures so they are actionable

[vlsi]

Here's jdk-unsafe:

@defaultMessage Uses default locale
java.lang.String#toLowerCase()
java.lang.String#toUpperCase()

I suggest adding the recommended action into the message.

@defaultMessage Use String#toLowerCase(Locale) to pass locale explicitly. For instance: .toLowerCase(Locale.ROOT)
java.lang.String#toLowerCase()
@defaultMessage Use String#toUpperCase(Locale) to pass locale explicitly. For instance: .toUpperCase(Locale.ROOT)
java.lang.String#toUpperCase()

An interesting case is

@defaultMessage Uses default locale or time zone
java.util.Calendar#<init>()

In fact, it uses both default locale and default timezone.
I added a timezone, rerun the check, and forbidden-apis was unhappy again on the same line, which was confusing for me for a couple of seconds.

WDYT?

[uschindler]

We can improve the already existing signaturs files. I will release v9.5 first then we can improve this. It is mainly adding texts.

Thanks for the suggestion.