docs(text): document whitespace-stripped indexing in Text (#4866)

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

Text.__init__ strips spaces/newlines from self.text and never creates
submobjects for whitespace, so slicing (text[a:b]) and the t2c/t2s/t2w/t2f/t2g
'[a:b]' syntax silently index into that stripped copy rather than the
original string, with no warning. Add a docstring callout explaining this and
pointing to substring-keyed t2c/etc. as the unaffected alternative.

Refs #4864

* docs(text): fix t2c/t2s/etc slice-index whitespace claim

Reviewer (chopan050) pointed out on PR #4866 that t2c/t2s/t2w/t2f/t2g
slice syntax (e.g. t2c={'[3:7]': RED}) indexes into the original text
with whitespace included, not the whitespace-stripped text as the
previous wording implied -- the opposite of how direct object slicing
(my_text[3:5]) behaves. Verified both behaviors empirically and
reworded the warning to describe each convention correctly.

---------

Co-authored-by: Francisco Manríquez Novoa <49853152+chopan050@users.noreply.github.com>

Author: SupernovaIa

manim/mobject/text/text_mobject.py

@@ -305,6 +305,29 @@ class Text(SVGMobject):
     Text objects behave like a :class:`.VGroup`-like iterable of all characters
     in the given text. In particular, slicing is possible.
 
+    .. warning::
+
+        Whitespace and newline characters are stripped internally and never
+        become their own submobject (there is nothing to render for them), so
+        slicing indices are interpreted differently depending on where they
+        are used, and the two conventions disagree with each other:
+
+        - Slicing the object itself (``my_text[3:5]``) indexes into the
+          rendered characters, i.e. the text *with whitespace removed*. For
+          ``Text("Hello world")``, index ``5`` refers to ``"w"``, not to the
+          space between the two words.
+        - The slice syntax in ``t2c``/``t2s``/``t2w``/``t2f``/``t2g``
+          (e.g. ``t2c={'[3:7]': RED}``) indexes into the *original* ``text``
+          argument, whitespace included. For ``Text("Hello World")``,
+          ``t2c={'[3:7]': RED}`` colors ``"l"``, ``"o"``, ``"W"`` (the space
+          at index 5 falls in range but has nothing to color), whereas
+          ``my_text[3:7]`` selects the 4 rendered characters ``"loWo"``.
+
+        When the substring you want to select is known in advance, prefer
+        keying ``t2c``/``t2s``/``t2w``/``t2f``/``t2g`` by that substring
+        directly (e.g. ``t2c={"world": RED}``), which matches by text search
+        instead of by index and is unaffected by this quirk.
+
     Parameters
     ----------
     text