Allow passing iterables to Scene.add() and improve comments for flatten_iterable_parameters()

[irvanalhaq9]

Overview: What does this pull request change?

This PR allows Scene.add() to accept iterable inputs directly, without requiring explicit unpacking. It also improves comments for flatten_iterable_parameters() in Scene.compile_animations() to clarify its purpose.

Motivation and Explanation: Why and how do your changes improve the library?

Currently, Scene.add() requires unpacking when passing an iterable of mobjects (e.g., self.add(*mobjects)). This PR modifies Scene.add() to use flatten_iterable_parameters(), allowing users to pass iterables directly (e.g., self.add(mobjects)). This improves consistency with Scene.play(), which already supports this behavior.
Additionally, comments for flatten_iterable_parameters() have been improved to clarify that it allows passing iterables—including generators—to self.play() without explicit unpacking.

self.play() can already accept both unpacked and non-unpacked iterables because it utilizes function flatten_iterable_parameters(). This PR applies the same approach to self.add(), allowing it to accept iterables directly without requiring unpacking.

Enhancement: Allow self.add() to accept iterables without unpacking

Current Behavior

Before this PR, self.play() already accepts both unpacked and non-unpacked iterables:

With unpacking:

class IterableWithSelfPlay(Scene): 
    def construct(self):
        st, sq, c = Star(), Square().shift(RIGHT*3), Circle().shift(LEFT*3)
        animations = [Create(st), Write(sq), FadeIn(c)]
        self.play(*animations, run_time=2)

Without unpacking:

class IterableWithSelfPlay(Scene): 
    def construct(self):
        st, sq, c = Star(), Square().shift(RIGHT*3), Circle().shift(LEFT*3)
        animations = [Create(st), Write(sq), FadeIn(c)]
        self.play(animations, run_time=2)

Both approaches work as expected.

However, self.add() requires unpacking:

class IterableWithSelfAdd(Scene): 
    def construct(self):
        grp = [Star(), Square().shift(RIGHT*3), Circle().shift(LEFT*3)]
        self.add(*grp)  # Unpacking is required

New Behavior

With this PR, self.add() can now accept iterables without unpacking:

class IterableWithSelfAdd(Scene): 
    def construct(self):
        grp = [Star(), Square().shift(RIGHT*3), Circle().shift(LEFT*3)]
        self.add(grp)  # Now works without unpacking

This change makes self.add() consistent with self.play() in handling iterables.

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested