docs: [File Instance] RunTimeException if file does not exist

[christianberkman]

Description
File::getSize and File::getSizeByUnit() may cause a RunTimeException if the file does not exist or an error occurs. From the original docblock my expectation was that the functions would return false on an error (e.g. file does not exist), but the called SplFileInfo::getSize() throws a runtime error if the file does not exist.

From the PHP docs

The filesize in bytes on success, or false on failure.
A RuntimeException will be thrown if the file does not exist or an error occurs.

From the PHP docs it is not quite clear to me when to expect an exception and when to expect false.

If return of false on any error is desired, I am happy to submit a PR to catch the exception.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

@christianberkman Thank you!