docs(text): document whitespace-stripped indexing in Text

[SupernovaIa]

Summary

Text strips whitespace/newlines from its internal self.text after construction, and never creates submobjects for whitespace characters (nothing to render). Since Text doesn't override __getitem__, slicing (text[a:b]) and the [a:b] slice syntax in t2c/t2s/t2w/t2f/t2g end up indexing into that whitespace-stripped copy — not the string the user actually passed in — with no warning or error.

This PR adds a docstring callout on Text explaining the behavior and pointing to substring-keyed t2c/etc. (matched by text search, unaffected by this) as the recommended alternative when the target substring is known ahead of time.

No behavior changes — docs only.

Fixes #4864

Test plan

• uv run pre-commit run --files manim/mobject/text/text_mobject.py passes (ruff, mypy, codespell)
• Text(...) still imports/constructs normally
• Docstring renders as expected (Text.__doc__ includes the new warning block)

[chopan050]

Thanks for the extra documentation!

I just made a local test, however, and the behavior for t2c and similar parameters does consider the whitespace, so the docstring has to mention this.

If text = Text("Hello World"), then, indeed, text[5] refers to the "W", not the whitespace. Thus, text[3:7] gets the 4 letters "l", "o", "W", "o". However, when using t2c={"[3:7]": RED}, this does consider the whitespace and paints "l", "o", "", "W" in red.

In other words:

class ColoringText(Scene):
    def construct(self):
        text = Text("Hello World", t2c={"[3:7]": RED})
        self.play(Write(text))
        self.play(text[3:7].animate.set_color(BLUE))
        self.wait()

starts with the "Hello World" text where "lo W" (without the second "o") is colored in red, and then "lo Wo" (with the second "o") gets colored in blue.

[SupernovaIa]

@chopan050 thanks for the careful test! You're right that the two paths disagree, so I've reworked the warning to document both conventions explicitly:

• Slicing the object itself (my_text[3:7]) indexes the rendered characters (whitespace stripped) → selects "loWo".
• The slice syntax in t2c/t2s/etc. (t2c={'[3:7]': RED}) indexes the original text argument, whitespace included → colors "l", "o", "W" (the space at index 5 falls in range but has nothing to render).

This matches the Text("Hello World", ...) example you described. Ready for another look whenever you have a moment 🙏

[chopan050]

LGTM!