StreamReader.ReadLine should document how newline at the end of the file is treated

[svick]

Describe the issue or suggestion

When StreamReader.ReadLine (and similar APIs, like File.ReadAllLines) reads a stream that ends in a newline, it doesn't produce a final empty line.

For example, a file containing the string "foo\nbar\n" is interpreted as having two lines, "foo" and "bar" (a file containing just "foo\nbar" is interpreted the exact same way).

This is common behavior (especially on Unix), but not universal, and it can be confusing, so I think it makes sense to briefly mention this in all affected APIs.

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