Minor: Add documentation explaining that initcap only works for ASCII

[alamb]

Which issue does this PR close?

• Related to Optimize performance of initcap function (~2x faster) #13691
• Related to Support unicode character for initcap function #13711

Rationale for this change

@tlm365 says: #13691 (comment)

@Dandandan you're right. I tested it with some unit tests for special characters (unicode), this PR's implementation doesn't work and the old implementation doesn't work either. But I think it makes sense at the moment since the initcap function is in datafusion::function::string not datafusion::function::unicode.

Perhaps, it would be better if we update the documentation that initcap is not supported for unicode characters yet and try to handle it if necessary in another PR.

What changes are included in this PR?

Update docs to reflect the caveat that init cap only works on ASCII strings

Are these changes tested?

by CI

Are there any user-facing changes?

Just docs, no functional change

[comphead]

lgtm thanks @alamb

[alamb]

Thank you for the review