strconv: document that an overflowing prefix reports ErrRange

[soreavis]

ParseInt and ParseUint stop reading s as soon as they detect the value
is out of range and return ErrRange at that point, so trailing invalid
digits are never examined. The doc says a string containing invalid
digits reports ErrSyntax, which does not hold when the numeric prefix
overflows first.

Document the current behavior rather than change it, as suggested on
the issue.

Fixes #78546

[gopherbot]

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