Document the text attribute mappings in the Text initializer

These were undocumented and it was difficult to find out what to provide
within them outside the bounds of the specific examples.

manim/mobject/text/text_mobject.py

@@ -317,6 +317,22 @@ class Text(SVGMobject):
     warn_missing_font
         If True (default), Manim will issue a warning if the font does not exist in the
         (case-sensitive) list of fonts returned from `manimpango.list_fonts()`.
+    t2c
+        A dictionary mapping substrings in the text to colors. These will be resolved
+        the same way they would by `set_color`. May also be provided as `text2color`.
+    t2f
+        A dictionary mapping substrings in the text to fonts. These will be resolved
+        the same way the `font` parameter is. May also be provided as `text2font`.
+    t2g
+        A dictionary mapping substrings in the text to gradients, specified as an iterable
+        of color strings. These will be resolved the same way `set_color_by_gradient` is.
+        May also be provided as `text2gradient`.
+    t2s
+        A dictionary mapping substrings in the text to slant values. May be
+        NORMAL (the default), ITALIC, or OBLIQUE. May also be provided as `text2slant`.
+    t2w
+        A dictionary mapping substrings in the text to font weight values, listed in
+        `manimpango.Weight`. May also be provided as `text2weight`.
 
     Returns
     -------
@@ -433,9 +449,9 @@ def __init__(
         font: str = "",
         slant: str = NORMAL,
         weight: str = NORMAL,
-        t2c: dict[str, str] = None,
+        t2c: dict[str, ParsableManimColor] = None,
         t2f: dict[str, str] = None,
-        t2g: dict[str, tuple] = None,
+        t2g: dict[str, Iterable[ParsableManimColor]] = None,
         t2s: dict[str, str] = None,
         t2w: dict[str, str] = None,
         gradient: tuple = None,