perldiag: Clarify uninit diagnostic explanation

[khwilliamson]

Most of the text for this change was contributed by @tonycoz

• This set of changes does not require a perldelta entry.

[bulk88]

Is the feature of running PP code without use warnings; being removed from the Perl 5 Lang? semi-joke

99.999% of cases of this message are bug.

%0.001 are a performance optimization (not anymore b/c someone forgot to put a {\n no warnings 'uninit'; ....`, and STDERR is very slow in milliseconds).

I would change the language of the sentence to say "this almost always is a bug".

Not the proposed language in the PR, which would concrete a law that says uninit warning is absolute "proof of bug [no other justification or tech reason supplied]". Auto casting to IV 0, and using value IOK 0 as input might have be original developers intent.

The uninit warning being drawn didn't create a bug where there was none before.

[Grinnz]

I think it is more indicating that it may be a mistake in the code, not in Perl. In other words, it may indicate a logic error or bug.

[khwilliamson]

Updated to incorporate @tonycoz wording; modified by me