What type `bound` parameter of `typing.TypeVar` accepts should be explained more clearly

Documentation

*Notes:

• Python 3.14.3

For T and A, what type is accepted is clearly explained in the example of typing.TypeVar while for S isn't explained clearly even though str itself is also accepted as shown below:

T = TypeVar('T')  # Can be anything
S = TypeVar('S', bound=str)  # Can be any subtype of str <- Here
A = TypeVar('A', str, bytes)  # Must be exactly str or bytes

So, S should be explained more clearly as shown below:

S = TypeVar('S', bound=str)  # Can be str or any subtype of str

Linked PRs

• gh-145525

从 typing.TypeVar 文档以及 issue 中链接的 T、S 和 A 示例开始。明确 bound=str 示例，明确说明 str 本身及其子类型均可接受，然后检查渲染后的文档，确保措辞一致。