Clarify is_end_of_document_line in docs

[TomJGooding]

The documentation for the DocumentNavigator says that a location "refers to a location, in document-space (in the raw document)".

I would expect the "end of line" in the example below to be at index 9. But the is_end_of_document_line is actually based on the length of the row, meaning it is off-by-one.

Perhaps what is meant by "end of line" just needs clarifying the docs, or this might be an unintended bug?

from textual.app import App, ComposeResult
from textual.widgets import TextArea

TEXT = "0123456789"


class ExampleApp(App):
    def compose(self) -> ComposeResult:
        yield TextArea.code_editor(TEXT)

    def on_mount(self) -> None:
        text_area = self.query_one(TextArea)
        expected_eol = (0, 9)
        assert text_area.navigator.is_end_of_document_line(expected_eol)


if __name__ == "__main__":
    app = ExampleApp()
    app.run()

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[TomJGooding]

Sorry I forgot to mention: I think the docstring might need correcting anyway here:

textual/src/textual/document/_document_navigator.py

Lines 107 to 108 in e1749ac

	Returns:
	True if and only if the document is on the last line of the document.

[darrenburns]

It's not off by one - I should maybe clarify in the docs though. The end of the line is the maximum position in a line the cursor can rest on. Since you need to be able to move the cursor beyond the end of the text (otherwise inserting new text would be impossible), the "end of document line" is len(document_line). It's not "index of the last character".

[TomJGooding]

Thanks Darren for the explanation, I think clarifying this in the docs is a good idea.

[TomJGooding]

Closed as completed by #4151 (thanks Darren!)

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